In a skillet over medium-high heat, melt the butter with the olive oil. Add the mushrooms and Worcestershire sauce. Cook for 5 minutes undisturbed until caramelized. Stir in beef broth, cooking until evaporated, about 8-10 minutes. Add garlic and black pepper, sauté for another 3-4 minutes. Set aside.

In a large bowl, combine ground beef, steak seasoning, and Worcestershire sauce. Mix gently to combine and shape into four equal patties. Let rest while grilling.

Preheat grill to medium-high (400°F). Grill patties for 4-5 minutes on one side until browned. Flip and grill for another 4-5 minutes. Add Swiss cheese during the last 1-2 minutes to melt.

Toast the brioche buns on the grill until golden, about 1-2 minutes. Assemble the burgers by placing patties on buns and topping with sautéed mushrooms.

Allow the assembled burgers to rest for 3-5 minutes before serving.