These French Onion Burgers are rich, savory, and totally indulgent. Juicy beef patties are topped with sweet caramelized onions and melty Gruyère cheese, all tucked into a soft bun slathered with creamy Dijon sauce. It’s like your favorite soup turned into a next-level burger!
In a large skillet over medium heat, melt the butter. Add sliced onions, salt, sugar, and thyme.
Cook the onions, stirring frequently, for 25–30 minutes until deeply caramelized. Add balsamic vinegar in the last few minutes if using. Set aside.
In a small bowl, mix mayonnaise, Dijon mustard, and lemon juice until smooth. Chill until ready to use.
Shape the ground beef into 4 equal patties. Season both sides with salt and pepper.
Heat a grill or skillet over medium-high heat. Cook the patties for about 3–4 minutes per side, or until desired doneness.
Place a slice of cheese on each patty during the last minute of cooking. Cover briefly to melt the cheese.
Toast burger buns until golden.
Assemble the burgers: spread Dijon mayo on each bun, place the cheesy patty on the bottom bun, top with caramelized onions, then close with the top bun.