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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.
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Prep Time 10 minutes
Cook Time 1 hour
Total Time 1 hour 10 minutes
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.