Start by gathering all your ingredients. Finely chop the mushrooms and slice the onions. Form the ground beef into patties, making an indentation in the center to prevent shrinking. Season with salt and pepper and set aside.

In a skillet, melt butter or heat olive oil over medium heat. Add the chopped mushrooms and sauté for 5-7 minutes until golden brown. Add onions halfway through cooking.

Heat your grill or skillet over medium-high heat. Cook the beef patties for 4-5 minutes on one side, then flip and cook for another 4-5 minutes until reaching 160°F.

Place a slice of Swiss cheese on each patty in the last few minutes of cooking to melt, covering with a lid to assist melting.

Slice your brioche buns in half and toast them on the grill or in a skillet for 1-2 minutes until golden brown.

Assemble the burger by placing each cooked patty on the bottom half of a toasted bun, topping with sautéed mushrooms and onions, and adding any additional toppings.