In a nonstick skillet, heat olive oil over medium-high heat. Add a single layer of mushrooms and cook, without stirring, for about 5 minutes or until mushrooms become red-brown on one side. Add salt and pepper, flip and cook 5 minutes longer, until other side is red-brown.
Remove mushrooms from pan and pour out any remaining liquid. Put bread in pan and warm both sides. With bread in pan, top slice with cheese and mushrooms; heat through.