Grease an 8-inch square baking dish and line the bottom with half of the bread slices.
Sprinkle bacon, mushrooms and onions over bread. Top with cheese and cover with remaining 4 bread slices.
In a medium bowl whisk together milk, eggs, salt and pepper; pour over bread. Cover. Refrigerate overnight.
Preheat oven to 350° F. Bake casserole, uncovered, for 1 hour or until golden brown and set Let stand 10 minutes before serving.Servings: 6