In a large skillet, heat butter and olive oil over medium heat. Add the thinly sliced onion and cook, stirring occasionally, until the onions are caramelized, about 20-25 minutes. Add sugar halfway through to enhance the caramelization. Set aside.

In a large mixing bowl, combine the ground beef, Worcestershire sauce, garlic powder, salt, and pepper. Mix until just combined. Divide and shape the mixture into 4 equal patties.

Preheat a grill or skillet over medium-high heat. Cook the patties for 4-5 minutes on each side, or until they reach your desired level of doneness.

During the last minute of cooking, place a slice of Gruyère or Swiss cheese on each patty and allow it to melt.

Toast the brioche buns on the grill or in the skillet until lightly browned.

Assemble the burgers by placing each patty on the bottom half of a brioche bun, topping with a generous portion of caramelized onions. Garnish with fresh thyme leaves if desired, and cover with the top half of the bun.

Serve immediately and enjoy the rich, savory flavors of these French Onion Soup Burgers.

Prep Time: 30 minutes