Gather all ingredients and slice the bread if not pre-sliced.
In a skillet, heat 1 tablespoon of oil over medium heat and sauté sliced mushrooms until golden brown, about 5 minutes. Season with salt and pepper.
Add ground beef to the skillet. Break apart with a spatula and cook until browned, about 5-7 minutes. Stir in Worcestershire sauce and season with salt and pepper.
Assemble the sandwich: Spread mayonnaise or garlic aioli on one slice of bread, layer with Swiss cheese, sautéed mushrooms, and ground beef, then top with another slice of Swiss cheese and another slice of bread spread with mayo.
In the skillet, melt 1 tablespoon of butter and grill the sandwich until golden brown and the cheese has melted, about 4-5 minutes per side. Press down slightly for better contact with the pan.
Remove from skillet, slice diagonally, and serve immediately.
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Notes
Store leftovers wrapped in aluminum foil or in an airtight container in the refrigerator for up to 2-3 days.