Mix ground beef with salt and pepper in a bowl. Shape into 4 patties, making a small dent in the center of each.

Heat butter and olive oil in a skillet over medium-high heat. Add mushrooms and onions, cooking until soft and golden brown. Season with salt and pepper, then set aside.

Preheat a grill or skillet over medium-high heat. Cook the patties for 4-5 minutes per side, or until they reach your preferred doneness.

During the last minute of cooking, place a slice of Swiss cheese on each patty. Cover the skillet or close the grill lid to melt the cheese.

Toast the brioche buns. Place the cooked patties on the buns, topping each with sautéed mushrooms and onions. Add any additional toppings as desired.

Enjoy immediately with your favorite sides.