PREHEAT oven to 400° F. Spray 10-inch oven-proof skillet with nonstick cooking spray.
COMBINE egg whites, eggs and dry milk in medium bowl; mix thoroughly. Stir in cheese, mushrooms, bell pepper, green onions, salt and black pepper. Pour mixture in prepared skillet.
BAKE for 30 to 35 minutes or until set in center. Cool on wire rack for 5 minutes before cutting into wedges.
FOR A LOWER-FAT FRITTATA
Substitute two 8-ounce cartons of egg substitute and 2 cups reduced-fat cheddar cheese for egg whites, eggs and cheddar cheese.