In a skillet, melt the butter over medium heat. Add the minced garlic and sauté for about 1 minute, until fragrant.
Add the sliced mushrooms to the skillet and cook, stirring occasionally, until they are tender and golden brown, about 5-7 minutes. Season with salt and pepper to taste. Remove from heat and set aside.
Heat a separate skillet or griddle over medium heat. Spread a thin layer of butter on one side of each slice of bread.
Place two slices of bread, buttered side down, on the skillet or griddle. Top each slice with a slice of cheese, then divide the cooked mushrooms evenly between the two slices.
Place the remaining slices of bread on top of the mushrooms, buttered side facing up.
Cook the sandwiches for 2-3 minutes on each side, or until the bread is golden brown and the cheese is melted.
Once cooked, remove the sandwiches from the skillet or griddle and let them cool for a minute or two before slicing.
Serve hot and enjoy!