Slice the mushrooms.
Finely chop the onion and garlic.
Melt butter in a pan and sauté the mushrooms, onion, and garlic.
Place cheese, sautéed mushrooms, onion, and garlic on the bread.
Cover with another slice of bread to make a sandwich.
Add butter to a pan and grill the sandwich.
Flip and grill until the bread is golden and crispy.