01 -
Heat the butter in a large skillet over medium heat. Add the thinly sliced onions and sprinkle with sugar. Cook, stirring occasionally, until the onions are deeply golden and caramelized, about 20-25 minutes. Deglaze the pan with a splash of beef broth if the onions begin to stick.
02 -
In a mixing bowl, season the ground beef with salt, pepper, and Worcestershire sauce. Mix thoroughly.
03 -
Shape the seasoned beef into 4 evenly sized patties. Cook in a skillet or on a grill to your preferred doneness.
04 -
If desired, mix minced garlic with softened butter. Spread on the buns and toast until golden brown.
05 -
Place a cooked beef patty on the bottom half of each bun. Add a generous portion of caramelized onions and top with a slice of Gruyère or Swiss cheese.
06 -
Broil the assembled burgers briefly until the cheese is melted and bubbly. Sprinkle fresh thyme over the melted cheese, top with the other half of the bun, and serve immediately.