Melt the butter in a skillet over medium heat.
Add the sliced mushrooms and garlic, season with salt and pepper.
Cook until the mushrooms are browned and tender, about 5-7 minutes.
Remove the mushroom mixture from the skillet and set aside.
Butter one side of each slice of bread.
Place the first slice of bread, buttered side down, in the skillet and layer with Swiss cheese and mushroom mixture.
Top with the second slice of bread, buttered side up.
Cook for about 4-5 minutes on each side, pressing down gently.
Once both sides are golden brown and the cheese has melted, remove from skillet.
Slice in half and serve warm.