Preheat oven to 350. Prepare a 13"x 9" pan with cooking spray and flour; set aside. In a saucepan, melt margarine. Blend with flour and cook for one minute. Gradually stir in milk and cook until thick. Add half teaspoon salt, black pepper, and cheese. Stir until cheese melts; set aside. In a skillet, cook onions and mushrooms until tender. Add cooked turkey, egg whites, eggs, and remaining salt. Cook over medium heat until eggs are set. Stir in flour-cheese mixture. Spoon into prepared pan. Sprinkle bread crumbs evenly over top. Cover and refrigerate overnight. Bake, uncovered, for 30 minutes, or until top is lightly browned.