Preheat oven to 400 degrees F. Melt butter in a small pan over medium heat. Saute onions and mushrooms until tender; set aside.. Place bottom halves of bread on a lined cookie sheet. Top each with 1 to 2 slices turkey, mushroom/onion mixture, and 1 to 2 slices cheese. Bake in preheated oven for about 5 minutes, or until cheese is melted. Remove from oven and pack one into your insulated lunchbox!