Classic Smash Burgers with Caramelized Onions

When the onion edges start turning gold, the skillet can look like it’s almost done—but a few pale, firm slices are enough to make a burger taste rushed. I let…

By Ava
•
November 10, 2026
•
Reading time: 10 min

Save to Pinterest



Print

Tip: save now, make later.

When the onion edges start turning gold, the skillet can look like it’s almost done—but a few pale, firm slices are enough to make a burger taste rushed. I let them cook until they’re soft and deeply golden, then use the same cast-iron skillet for the buns and patties after wiping it clean. That slow start gives these classic smash burgers a sweet onion layer without needing a sauce or extra toppings.

The burgers are built from eight small portions of 80% lean ground beef, stacked two to a bun with American cheese. That gives you the crisp browned edges of a smash burger in a satisfying double stack, with a slice of cheese melting over each patty. The buns get a quick butter toast in the skillet, so the finished burger has a golden, sturdy base under all those layers.

Why a Double Smash Stack Beats One Thick Patty Here

A thick burger and a smash burger solve different problems. A thicker patty gives you a taller, meatier center, while thin smash patties put more of the beef against the hot skillet. That extra browned surface is what gives each bite its crisp edges, and stacking two patties creates height without losing the defining smash-burger texture.

There’s a useful balance in this particular stack: the beef brings savory, browned edges, the American cheese melts between the layers, and the slowly cooked onion adds sweetness. Two thin patties let those parts stay distinct instead of burying the onion and cheese inside one thick center. You still get a substantial burger, but the bite stays layered rather than tasting like a single dense piece of beef.

The 80% lean ground beef also suits this style. Its fat helps the patties brown while they cook quickly, and the thin shape means there’s no need to wait for a thick center to finish. That’s why I portion the beef for a double stack rather than shaping four larger patties.

What the Beef, Cheese, and Onion Each Bring to the Stack

• 80% lean ground beef — The fat keeps the thin patties juicy and helps them brown. Use fresh beef and handle it gently; leaner ground beef can work, but the patties will be less rich and more prone to drying out. If 80% lean isn’t available, 85% lean is a workable swap with a less fatty result.
• American cheese — It melts smoothly over a thin patty in a short cook, making it a natural fit here. Cheddar can stand in, but it has a sharper taste and won’t melt as fluidly. The quality difference between standard and pricier American cheese is less important than using slices that fit the patties.
• Yellow onion — Yellow onion turns soft and sweet over the long, gentle cook. Slice it thinly and evenly so the pieces soften at roughly the same pace. White onion can replace it, though the finished onions will taste a little sharper.
• Unsalted butter and neutral oil — Butter gives the onions and toasted buns a rounded flavor, while the neutral oil helps the onion mixture cook without relying on butter alone. If you’re out of neutral oil, another neutral cooking oil is the closest swap. The exact type matters less than avoiding an oil with a strong flavor.
• Hamburger buns — A basic, sturdy bun is enough; the skillet toast gives the cut sides their golden finish. There’s no need to spend extra on an elaborate bakery bun, which can compete with the thin patties and onions.

The Skillet Sequence That Gets Every Layer Ready

Let the onions turn deeply golden

Melt the butter with the neutral oil in a 12-inch cast-iron skillet over medium-low heat. Add the thinly sliced onion and kosher salt, then cook for 35 to 45 minutes, stirring often, until the onion is soft and deeply golden. If it starts browning too quickly, lower the heat so it has time to soften instead of scorching. Transfer the finished onions to a bowl before moving on.

Toast the buns while the skillet is clean

Heat the oven to 200°F and set a rimmed baking sheet inside to warm. Wipe the skillet clean, then spread the cut sides of the buns with softened butter. Toast them cut-side down over medium heat in batches for 1 to 2 minutes, until golden, and set them aside. Working in batches keeps the buns in contact with the skillet instead of crowding them.

Portion the beef and heat the pan

Divide the beef into eight equal portions and gently roll each one into a loose ball. Heat the skillet over medium-high for 2 to 3 minutes, until it’s very hot. Place two beef balls in the skillet with plenty of space between them; the room lets you press each one flat without pushing it into the other.

Smash, season, and melt the cheese

Use a stiff spatula to firmly smash each ball into a thin patty, then season each with 1/8 teaspoon kosher salt and a pinch of black pepper. Cook for 1 to 2 minutes, until the edges are browned and crisp. Scrape under each patty to flip it, add a slice of American cheese, and cook for 30 to 60 seconds, until the cheese melts and the beef reaches 160°F when checked with an instant-read thermometer inserted from the edge. Transfer the patties to the warm baking sheet, repeat with the remaining beef and cheese, then stack two patties on each bun and top with the onions.

Three Simple Ways to Change the Burger Stack

Questions That Come Up While Making Smash Burgers

Classic Smash Burgers with Caramelized Onions

Smash burgers with caramelized onions pair thin, crisp-edged beef patties and melted American cheese with soft, deeply golden onions. Toasted buttered buns hold two patties each for a classic cast-iron skillet burger.

Print Recipe Pin Recipe

Prep Time 10 minutes mins

Cook Time 1 hour hr

Total Time 1 hour hr 10 minutes mins

Servings: 4 servings

Course: Dinner

Cuisine: American

Calories: 850

Ingredients Equipment Method Notes

Ingredients

  

For the caramelized onions

• 1 tbsp unsalted butter
• 1 tbsp neutral oil
• 1 yellow onion large, thinly sliced
• 0.25 tsp kosher salt

For the burgers

• 4 hamburger buns
• 2 tbsp unsalted butter softened
• 1.5 lb ground beef 80% lean
• 1 tsp kosher salt
• 0.5 tsp black pepper
• 8 slices American cheese

Equipment

• 1 12-inch cast-iron skillet
• 1 rimmed baking sheet

Method

 

Cook the onions

1. Melt the butter with the oil in a 12-inch cast-iron skillet over medium-low heat.
2. Add the onion and 1/4 teaspoon salt and cook for 35-45 minutes, stirring often, until soft and deeply golden.
3. Lower the heat if the onion starts browning too quickly, then transfer the onions to a bowl.

Prepare the buns

1. Heat the oven to 200°F and set a rimmed baking sheet inside to warm.
2. Wipe the skillet clean.
3. Spread the cut sides of the buns with the softened butter.
4. Toast the buns cut-side down over medium heat in batches for 1-2 minutes, until golden.
5. Set the toasted buns aside.

Cook the burgers and assemble

1. Divide the beef into 8 equal portions and gently roll each into a loose ball.
2. Heat the skillet over medium-high heat for 2-3 minutes, until very hot.
3. Place 2 beef balls in the skillet with plenty of space between them.
4. Smash each ball firmly with a stiff spatula into a thin patty.
5. Season each patty with 1/8 teaspoon salt and a pinch of pepper.
6. Cook the patties for 1-2 minutes, until the edges are browned and crisp.
7. Scrape under each patty to flip it.
8. Top each patty with a slice of cheese.
9. Cook for 30-60 seconds, until the cheese melts and the beef reaches 160°F when checked with an instant-read thermometer inserted from the edge.
10. Transfer the patties to the warm baking sheet and repeat with the remaining beef and cheese.
11. Stack 2 patties on each bun.
12. Top the burgers with the caramelized onions.

Notes

For crisp edges, leave plenty of space between the beef balls and use a very hot skillet. Refrigerate the cooked components separately in airtight containers for up to 3 days; cooked patties and onions can be frozen for up to 2 months. Use gluten-free buns for a gluten-free option.