If you're craving a melt-in-your-mouth sandwich with a rich twist, this Buttery Roast Beef Sliders with Caramelized Onions Recipe will be your new go-to. Soft buns, savory beef, gooey cheese, and sweet, jammy onions come together in a bite-sized comfort explosion.

Two toasted sandwich buns with a golden brown color and sprinkled with sesame seeds and seasoning sit stacked on a dark wooden board. The bottom bun holds a layer of melted cheese that drapes over the edges, topped with caramelized onions and slices of cooked meat peeking out from under the cheese. The top bun rests slightly off-center on the filling, showing more melted cheese and caramelized onions oozing from the sides. The background is dark, and the board contrasts with the textures of soft buns, shiny melted cheese, and juicy meat and onions. Photo taken with an iphone --ar 2:3 --v 7

Jump to:

Why You'll Love This Recipe
Ingredients & Why They Work
Make It Your Way
Step-by-Step: How I Make Buttery Roast Beef Sliders with Caramelized Onions Recipe
Top Tip
How to Serve Buttery Roast Beef Sliders with Caramelized Onions Recipe
Make Ahead and Storage
Frequently Asked Questions:
Final Thoughts
Buttery Roast Beef Sliders with Caramelized Onions Recipe

Why You'll Love This Recipe

I honestly can’t get enough of these sliders. Every element works in harmony—from the creamy garlic aioli to the warm au jus sauce that makes dunking absolutely irresistible. They’re perfect for game day, parties, or anytime you want a hearty snack that feels downright indulgent.

Perfect Party Food: These sliders are great for feeding a crowd and easy to make in batches.
Flavor Explosion: The sweet caramelized onions balance beautifully with the savory roast beef and melty cheese.
Hands-On Comfort: The garlic butter-brushed buns bring a rich, buttery finish that’s super satisfying.
Dippable Au Jus: Warm, flavorful au jus makes every bite even better when dipped.

Ingredients & Why They Work

When you prep this recipe, selecting quality ingredients really boosts the final taste. For instance, choosing soft Hawaiian slider buns adds a slight sweetness that pairs nicely with the caramelized onions. And trying a mix of melty cheeses like provolone and mozzarella gives every bite that ooey-gooey factor.

Flat lay of a stack of 3 large yellow onions, thinly sliced; a small white ceramic bowl with a smooth pale yellow egg yolk; a single whole garlic clove, white and firm; a small white ceramic bowl filled with light golden olive oil; a small white ceramic bowl with bright yellow fresh lemon juice; a pat of unsalted butter, pale creamy and rectangular; a small white ceramic bowl of brown sugar; a small white ceramic bowl of coarse kosher salt; a small white ceramic bowl of deep brown beef broth; a small white ceramic bowl of dark Worcestershire sauce; a handful of thinly sliced deli roast beef, deep red with marbled fat; twelve soft, round slider buns with a glossy golden-brown crust sliced in half; twelve smooth, pale provolone cheese slices; a small white ceramic bowl of shredded white mozzarella cheese; a small white ceramic bowl with poppy seeds; a small white ceramic bowl with white sesame seeds; a small white ceramic bowl with dried minced onion; a small white ceramic bowl with garlic powder; a few whole brown eggs with clean shells arranged symmetrically nearby, placed on a clean white marble surface, soft natural light, photo taken with an iPhone, professional food photography style, fresh ingredients, white ceramic bowls, no bottles, no duplicates, no utensils, no packaging --ar 2:3 --v 7 --p m7354615311229779997

Unsalted Butter: Used both for caramelizing onions and making the garlic butter sauce, it adds richness without overpowering.
Olive Oil: Helps sauté the onions gently to develop their sweetness.
Yellow Onions: Their natural sugars caramelize beautifully, bringing a deep jammy flavor.
Kosher Salt: Essential for seasoning and enhancing all the savory elements.
Brown Sugar: A small touch speeds up caramelization and balances the onion's bite.
Beef Broth: Used for deglazing the pan during onion cooking and as the base for the au jus sauce.
Egg Yolk: The foundation for homemade garlic aioli, giving it creamy texture and richness.
Dijon Mustard: Adds a subtle tang and depth to the aioli.
Garlic: Infuses both the aioli and garlic butter sauce with savory punch.
Lemon Juice: Brightens the aioli, balancing the richness with fresh acidity.
Light Olive or Neutral Oil: Used to emulsify the aioli with a delicate flavor.
Better than Bouillon Beef Base: Concentrated flavor that makes the au jus rich and meaty.
Worcestershire Sauce: Adds a complex umami kick in both the au jus and garlic butter sauce.
Slider Buns (Hawaiian preferred): Soft, slightly sweet buns that soak up juices without falling apart.
Deli Roast Beef: Thin slices of tender beef provide that classic sandwich protein punch.
Melty Cheese (Provolone, Swiss, or Muenster): Melts smoothly over the beef for ultimate cheesiness.
Shredded Mozzarella: Sprinkled on top for an extra layer of gooey melt.
Poppy Seeds & White Sesame Seeds: Give the garlic butter-brushed tops a subtle nutty crunch and visual appeal.
Garlic Powder and Dried Minced Onion: Season the garlic butter sauce creating layers of flavor.
Freshly Cracked Black Pepper: Adds a little heat and complexity finishing off the garlic butter sauce.

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Note: You'll find the complete list of ingredients, along with their exact measurements, in the printable recipe card at the bottom of this post.

Make It Your Way

One of the best parts about this Buttery Roast Beef Sliders with Caramelized Onions Recipe is how easy it is to customize to fit your taste buds or occasion. Whether you want to switch up the cheese, add a fresh crunch, or lighten it up, these sliders are a fantastic base for your creativity.

Cheese Swap: I love trying different melty cheeses like provolone, Swiss, or muenster for a flavor twist. Sometimes I mix in sharp cheddar for a bolder bite that pairs beautifully with the sweet onions.
Make It Vegetarian: For a meatless option, swap the roast beef with grilled portobello mushrooms or a hearty plant-based deli slice. The caramelized onions and garlic aioli keep that rich umami punch intact.
Speed It Up: When time is tight, skip the slow caramelized onions and jazz up regular mayo with garlic, lemon, and mustard for a similar flavor boost. The sliders still come together in about 30 minutes!
Add Freshness: Topping the sliders with a few slices of pickles or fresh arugula just before serving adds a lovely bite and balance to all the buttery richness.

Step-by-Step: How I Make Buttery Roast Beef Sliders with Caramelized Onions Recipe

A close-up shows a woman's hand holding a sandwich with three layers: a toasted, sesame and poppy seed covered top bun in golden brown with a shiny glaze; inside there are layers of caramelized onions in dark brown with a glossy texture, melted cheese dripping in light yellow, and thin slices of cooked meat in pink with browned edges; below is the bottom bun matching the top in texture and color. The sandwich is being dipped into a white bowl filled with thick, brown dipping sauce that is dripping down the sides and splashing on the wooden surface below. In the background, there are more stacked sandwiches with the same layers on a wooden board, set against a white marbled textured surface. Photo taken with an iphone --ar 2:3 --v 7

Step 1: Slowly caramelize the onions to golden perfection

Start by melting 2 tablespoons of unsalted butter with 1 tablespoon of olive oil in a large skillet over medium-low heat. Add your thinly sliced onions along with ½ teaspoon kosher salt and 1 teaspoon brown sugar (this little touch helps speed up caramelization). The trick here is patience—let the onions cook low and slow, stirring occasionally, until they’re soft, deeply golden brown, and irresistibly jammy. This takes about 45 minutes, but the sweet, rich flavor they develop is absolutely worth it. Pour in 2 tablespoons beef broth to deglaze the pan, scraping up those flavorful browned bits, then set the onions aside.

Step 2: Whisk up a creamy homemade garlic aioli

While your onions are caramelizing, you can make the garlic aioli. In a small bowl, whisk together 1 large room temperature egg yolk, 1 teaspoon Dijon mustard, 1 finely grated or minced garlic clove, and 1 tablespoon fresh lemon juice until smooth. Then, slowly stream in ½ cup of light olive oil (or any neutral oil) while whisking or using an immersion blender until the mixture thickens luxuriously. Add kosher salt to taste and set aside. If you're short on time, simply mix garlic, mustard, lemon juice, and salt into store-bought mayo for a quick version that tastes great too.

Step 3: Simmer savory au jus sauce for dipping

Next, melt ¼ cup unsalted butter in a medium saucepan over medium heat. Stir in 1 tablespoon Better than Bouillon Beef Base, 2 cups beef broth, and 2 teaspoons Worcestershire sauce. Let everything simmer gently for 20 minutes until the liquid reduces by half, thickens slightly, and turns a rich mahogany color. Keep this warm when you’re ready to serve—this au jus is pure comfort in a dipping cup!

Step 4: Toast buns and brush with flavorful garlic butter

Preheat your oven to 350°F (180°C / Gas Mark 5). Slice 12 slider buns in half horizontally but keep the tops attached to avoid scaling apart. Arrange the bottom halves, cut side up, on a parchment-lined baking sheet. Lightly brush them with your choice of melted butter or some of the garlic aioli. Toast these buns for 5 minutes until they turn a gentle golden hue—this adds a lovely crunch and helps them hold up against all the juicy toppings.

Step 5: Layer roast beef, gooey cheeses, and caramelized onions

Now the fun part: assembly! Spread a generous layer of your homemade garlic aioli over the toasted bottom buns. Pile on thinly sliced deli roast beef (about ½ pound, layered 2 to 3 deep) for that juicy, tender core. Place 12 slices of your preferred melty cheese (provolone, Swiss, or muenster are fab) over the beef, then sprinkle ½ cup shredded mozzarella for extra creamy meltiness. Finally, spoon an even 2 cups of your rich caramelized onions atop each slider before crowning with the top bun halves.

Step 6: Brush tops with garlic butter sauce and bake to golden bliss

In a small bowl, combine 6 tablespoons melted unsalted butter with 1 teaspoon Worcestershire sauce, 1 tablespoon poppy seeds, 1 tablespoon white sesame seeds, ½ tablespoon garlic powder, ½ tablespoon dried minced onion, ½ teaspoon kosher salt, and freshly cracked black pepper to taste. Brush this fragrant, nutty garlic butter sauce generously over the slider tops. Cover the tray loosely with foil and bake for 10 minutes. Then remove the foil and bake for another 10 minutes, allowing the tops to achieve a beautiful golden brown and the cheese to flow into gooey perfection.

Step 7: Serve warm with au jus for dipping and enjoy!

Once out of the oven, slice carefully between the buns to separate each slider. Serve your Buttery Roast Beef Sliders with warm bowls of the au jus sauce on the side for dunking. The combination of buttery toasted buns, rich roast beef, sweet onions, and savory dip is pure comfort food magic. Gather your friends or family, and get ready for loads of satisfied smiles!

Top Tip

Making these sliders just right is all about attention to detail. Here are some tidbits from my kitchen that really elevate the Buttery Roast Beef Sliders with Caramelized Onions Recipe to next-level deliciousness.

Slow-Caramelize Those Onions: Patience is your best friend here. Cooking the onions over low heat for the full 45 minutes ensures that deep jammy sweetness that perfectly balances the savory roast beef.
Brush the Buns Generously: Don’t be shy with that garlic butter mix—brushing the buns well before baking lends them so much buttery, flavorful richness and a beautiful golden crust.
Layer Cheese for Maximum Melt: I like layering a slice of melty cheese like provolone with a sprinkle of shredded mozzarella on top—the combo melts into gooey perfection that wraps around the roast beef.
Don’t Skip the Au Jus: That warm dipping sauce seals the deal. It adds moisture and a savory depth that makes every bite incredible.

How to Serve Buttery Roast Beef Sliders with Caramelized Onions Recipe

The image shows five pieces of bread spread with a creamy white mixture that has small green flecks, all placed neatly in a baking tray. The bread pieces vary in size and shape, with three longer slices arranged horizontally on the top and middle, and two smaller slices on the lower right side. The creamy spread is unevenly applied, with some areas thicker and others thinner, showing a rough texture on the bread underneath. The baking tray has a light beige tone, and the background surface has been changed to a white marbled texture. Photo taken with an iphone --ar 2:3 --v 7

Garnishes

For a beautiful presentation and extra flavor, try topping the sliders with a few fresh arugula leaves or a sprinkle of finely chopped fresh parsley. A light dash of cracked black pepper atop each slider right before serving also adds a nice touch. If you like a little heat, a few thin slices of pickled jalapeños make a delightful contrast to the buttery richness.

Side Dishes

Classic pairings here make the meal feel complete. Crispy baked sweet potato fries or classic shoestring fries are fantastic for that satisfying crunch. For something greener, a crisp Caesar salad or tangy coleslaw complements the richness of the sliders. If you want to keep things cozy, creamy mashed potatoes or a warm vegetable medley work just as well.

Make Ahead and Storage

Storing Leftovers

Once you’ve devoured what you want, store any leftover sliders in an airtight container in the refrigerator. They’ll keep well for up to 3 days, giving you plenty of time to enjoy a second round of these buttery delights.

Freezing

If you want to stash some for later, it’s best to freeze the sliders before baking and adding the caramelized onions and garlic butter topping. Wrap them tightly in plastic wrap and then foil to prevent freezer burn. When you’re ready, thaw overnight in the fridge and finish the assembly and baking steps fresh for best results.

Reheating

For leftovers, reheat in the oven at 375°F for about 10 minutes. This method keeps the buns toasty and prevents sogginess, and gently melts the cheese again so you get that fresh-baked joy all over again.

Frequently Asked Questions:

Can I use premade caramelized onions instead of making them from scratch?

Yes! Premade caramelized onions can be a convenient shortcut. Just warm them up before assembling the sliders to maintain that rich, jammy texture and flavor that homemade onions provide.

Is there a way to make this recipe quicker?

Absolutely. For a 30-minute version, skip the slow caramelized onions and jazz up mayonnaise with garlic, lemon, and mustard to mimic the aioli. The sliders will still be delicious and flavorful, just less sweet and deep in onion flavor.

What type of cheese works best for these sliders?

Melty cheeses like provolone, Swiss, or muenster work beautifully because they melt evenly and add a creamy texture that complements the beef and onions perfectly. Layering with shredded mozzarella adds extra gooeyness.

Can I make these sliders vegetarian?

While this recipe centers on roast beef, you can easily swap the beef for grilled mushrooms or a plant-based deli slice. Keep the caramelized onions and garlic aioli for that same rich, indulgent flavor profile.

Final Thoughts

Making Buttery Roast Beef Sliders with Caramelized Onions is one of those kitchen adventures that rewards you with pure comfort and crowd-pleasing flavor. Whether you’re serving them up for a casual get-together or savoring them as a cozy snack, these sliders strike a perfect balance of savory, sweet, and buttery goodness. I hope my tips and tricks inspire you to make them your own and enjoy every melty, juicy bite!