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A cheeseburger with melted cheese, caramelized onions, greens, and sauce on a toasted bun sits on a plate.
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French Onion Burgers

Deep caramelized onion jam, melted Gruyère, and a yellow mustard smash crust — everything that makes French onion soup great, now on a burger. Make the jam ahead and the rest comes together in five minutes of hot cast iron.
Prep Time20 minutes
Cook Time1 hour 10 minutes
Total Time1 hour 30 minutes
Course: Main Course
Cuisine: American
Servings: 4 serving (1 burger burger per person)
Calories: 797kcal
Author: Brittany Lauren

Equipment

Ingredients

For the Onion Jam

  • 2 large yellow onions thinly sliced
  • 2 tablespoons unsalted butter
  • 1 teaspoon kosher salt
  • ½ teaspoon sugar optional
  • 1 teaspoon Worcestershire sauce
  • ¼ cup beef broth or beef stock
  • Freshly cracked black pepper

For the Thyme Aioli

  • ½ cup full-fat mayo
  • ½ teaspoon fresh lemon juice
  • 1 tablespoon fresh thyme leaves
  • ½ teaspoon freshly cracked black pepper

For the Burgers

  • 1 lb ground beef 80/20 chuck
  • Kosher salt
  • 2 to 3 teaspoons yellow mustard classic, not Dijon
  • Freshly cracked black pepper
  • 4 slices Gruyère cheese
  • 4 soft burger buns brioche or potato
  • 2 cups arugula
  • Neutral oil or beef tallow for the griddle

Instructions

Make the Onion Jam

  • Melt butter in a wide skillet over medium-low heat. Add onions and salt, stir to coat. Cook 45 to 60 minutes, stirring occasionally, until deeply browned, jammy, and reduced. Add Worcestershire and beef broth. Simmer until thick and glossy, 3 to 5 minutes. Finish with black pepper. Set aside or refrigerate up to 3 days.
    Sautéed caramelized onions in a black pan with a white handle, placed on a textured brown surface.

Make the Thyme Aioli

  • Whisk together mayo, lemon juice, thyme, and black pepper until smooth. Refrigerate until ready to use, up to 2 days ahead.
    A metal whisk rests in a glass bowl filled with a creamy, thyme aioli sauce on a textured brown surface.

Prep the Beef

  • Divide the ground beef into 4 equal portions and gently press each one into a loose patty, about 4½ to 5 inches wide - don't overwork it. Lightly spread ½ to ¾ teaspoon of yellow mustard on one side of each patty.
    Two raw hamburger patties and two raw meatballs on a wooden board, one patty topped with a thin layer mustard and another with a small amount of mustard.

Heat the Skillet

  • Heat cast-iron skillet or flat-top over high heat until screaming hot, 3 to 5 minutes. Lightly oil the surface.

Smash and Sear

  • Place the patties mustard-side down. Season the top with kosher salt. Cook undisturbed 2 to 3 minutes until a deep mahogany crust forms and releases cleanly. If it sticks, wait 10 to 15 more seconds.
    Raw hamburger patty placed in the center of a black frying pan with a white handle, on a textured brown surface.

Flip and Melt

  • Flip each patty, season with black pepper, top with Gruyère. Cover with a dome or lid and cook 60 to 90 seconds until cheese is melted and internal temp reads 160°F.
    A single hamburger patty topped with melted cheese sits in the center of a black skillet with a white handle on a textured brown surface.

Toast and Assemble

  • Toast buns cut-side down in burger fat. Spread aioli on the bottom bun. Layer arugula, patty, then a generous spoonful of onion jam. Top and serve immediately.
    Open burger on a wooden board with toasted buns, mayonnaise, arugula, melted cheese, and caramelized onions on a beef patty.

Notes

  • Don't season the beef before forming. Salt breaks down proteins early and causes steaming rather than searing.
  • Make the onion jam 1 to 3 days ahead. Flavor improves overnight. Reheat it with a splash of water or beef broth to loosen if needed before buulding the burgers - you want the onions warm when you bite into the smash burgers. 
  • Thyme aioli can be made up to 2 days ahead.
  • The onion jam keeps refrigerated for up to 5 days.
  • Store components separately. Don't assemble before storing.

Nutrition

Serving: 1serving | Calories: 797kcal | Carbohydrates: 29g | Protein: 34g | Fat: 60g | Saturated Fat: 21g | Polyunsaturated Fat: 15g | Monounsaturated Fat: 19g | Trans Fat: 2g | Cholesterol: 138mg | Sodium: 1356mg | Potassium: 544mg | Fiber: 2g | Sugar: 7g | Vitamin A: 785IU | Vitamin C: 9mg | Calcium: 410mg | Iron: 5mg