In a 10-inch nonstick skillet, heat oil over medium heat. Add potatoes and onion; cook 5 to 7 minutes, stirring occasionally, or until potatoes are cooked through and lightly browned.
Reduce heat to medium-low. In a medium bowl, whisk together eggs, egg whites, parsley, Parmesan cheese, and pepper. Pour egg mixture into pan, evenly spreading to coat bottom. Cover and cook 15 to 20 minutes, or until center is cooked through.