12 slices white bread, cut into 1/2 inch cubes
2 tablespoons butter
1 onion, diced
1 cup sliced mushrooms
2 teaspoons salt
1 teaspoon black pepper
1/3 cup milk
4 tablespoons mayonnaise
Place sliced bread cubes in large bowl; add butter or margarine, diced onion, mushrooms, salt, pepper, milk and mayonnaise. Mix well. Transfer mixture to large bowl. Cover; refrigerate several hours or overnight.
Preheat oven to 350 degrees F (175 degrees C).
Place bread cubes in 9 inch springform pan; grease bottom of springform pan with cooking spray. Place apple on bread. Brush tops of bread cubes with egg whites.
Bake in preheated oven for 30 minutes. Remove bread cubes, cool before cutting into slices.