Heat oven to 350°F. Spray 8-inch square (2-quart) glass baking dish with nonstick cooking spray.
In medium bowl, combine egg whites, milk and flour; beat until well blended. Place half of the chiles in sprayed baking dish. Cover with half of the egg mixture. Reserve 1/2 cup of each cheese for topping. Sprinkle mixture with half of each remaining cheese. Repeat layers. Pour tomato sauce over top layer of cheese.
Bake 350°F. for 20 minutes. Sprinkle with reserved cheese. Bake an additional 25 to 30 minutes or until edges are bubbly and cheese is melted.