Preheat oven to 325 &F. Take softened butter and spread on bread slices. Spread buttered bread slices on bottom of a buttered 9x9x2 inch baking pan. In a large skillet, melt the 1/4 cup butter or margarine and saute the sliced mushrooms, celery, red peppers and onions till brown, about 8 minutes. Combine cooked mushroom mixture, mayonnaise, salt, and pepper. Spread over bread slices. Top with remaining bread slices, cubed. Combine eggs and milk; pour over bread. Refrigerate 1 hour or longer. When ready to bake, pour soup over casserole. Bake at 325 &F for 50 minutes; add cheese, if desired, and bake 10 minutes longer. Let stand 10 minutes before serving. Serves 8-10