1. Preheat oven to 350°.
2. Heat oil in a large nonstick skillet over medium heat. Add onion; cook 15 minutes or until golden, stirring frequently. Set aside.
3. Combine eggs, egg whites, cheese, milk, salt, and black pepper in a medium bowl, stirring with a whisk.
4. Pour egg mixture into skillet with onion; stir to evenly distribute eggs. Top with bell pepper in a bicycle-spoke pattern. Cook 2 minutes or until edges begin to set. Wrap handle of skillet with foil; transfer to oven.
5. Bake, uncovered, at 350° for 15 minutes or until frittata is set. Cut into 6 wedges.
Note: Nutritional analysis includes Sugars 3g.