- In 8 or 9-inch nonstick ovenproof skillet, heat oil over medium heat.
- Add all onions and cook, stirring occasionally, until softened, 5 minutes.
- Add potatoes to skillet, spreading to cover bottom of pan.
- In medium bowl, whisk together eggs, egg whites, milk, salt and pepper.
- Reduce heat to low and pour egg mixture over vegetables, covering evenly.
- Cover and cook until egg mixture has set around edges but center is still liquid, 6 to 8 minutes.
- Preheat broiler.
- Scatter cheese over surface.
- Place skillet under broiler and cook until lightly golden, about 1 minute.
- Cut the frittata into 4 wedges and serve warm.