In a mixing bowl, combine ground beef, grated onion, minced garlic, and Worcestershire sauce. Mix gently until combined. Divide the mixture into four equal portions and shape into patties slightly larger than the brioche buns.

Heat a skillet or grill pan over medium-high heat. Cook the patties for 3–4 minutes per side or until cooked to your preferred doneness. Add a slice of cheese to each patty during the last minute of cooking to melt.

In a separate skillet, melt the butter over medium heat. Add the sliced mushrooms and cook for 4–5 minutes, stirring occasionally, until golden and softened.

Stir in the minced garlic, salt, and pepper. Cook for another minute, then remove from heat.

Lightly butter and toast the brioche buns on a skillet or grill until golden.

On the bottom bun, layer the cooked patty with melted cheese, a generous spoonful of sautéed mushrooms, and any additional toppings or condiments you like. Top with the other half of the bun.