Melt butter in a small skillet over medium heat. Saute mushrooms and onion 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 top each sandwich with olives, tomato and lettuce. Place top half of bread on each sandwich and serve.