In a large skillet, melt 1 tablespoon of butter over medium-low heat. Add sliced onions and a pinch of salt. Cook slowly for 20-30 minutes, stirring occasionally, until deeply golden brown and sweet.

In a separate skillet, melt 1 tablespoon of butter over medium-high heat. Add mushrooms and cook until browned, about 5-7 minutes. Stir in Worcestershire sauce, salt, and pepper, and cook for 1 more minute. Set aside.

In a bowl, gently mix ground beef, garlic powder, 1 tsp Worcestershire sauce, salt, and pepper. Form into two thin, oval-shaped patties. Cook in a skillet over medium-high heat for 3-4 minutes per side, until cooked through.

Butter one side of each slice of bread. Place two slices, butter-side down, in a clean skillet over medium heat. Layer with one slice of Swiss cheese, a beef patty, caramelized onions, sautéed mushrooms, and another slice of Swiss cheese.

Top with the remaining bread slices, butter-side up. Grill for 3-4 minutes per side, pressing gently, until the bread is golden brown and the cheese is fully melted.

Let the melt rest for a minute before slicing in half and serving immediately.