In large skillet melt butter and saute onions and mushrooms in butter.Place onions and mushrooms in 2 gt. Casserole dish. Sprinkle with shredded cheese. Combine soup, soy sauce evaporated milk and pour over cheese.Layer bread on top of that and top with slices of cheese..cover and refrigerate 4 hrs. Bake fro 30 min. Uncovered for 20 min.May substitute your favorite cheese.