Melt butter in a skillet over medium heat. Add sliced mushrooms and cook for 5–7 minutes until golden and tender, stirring occasionally. Set aside.
Mix ground beef with Worcestershire sauce, salt, and pepper. Shape into patties slightly larger than the bread.
Cook in the same skillet over medium-high heat, 3–4 minutes per side, until fully cooked. Set aside to rest.
Spread mayo or garlic aioli on each bread slice. Layer Swiss cheese, beef patty, mushrooms, and another slice of cheese. Top with the second bread slice, mayo-side down.
Heat oil in a skillet over medium heat. Place sandwiches in the pan, press gently, and grill for 3–4 minutes per side until golden and crisp, with melted cheese.
Slice in half and serve hot while the cheese is melty and the bread is crispy.