Preheat broiler to high. In a 6-inch skillet, heat the olive oil over medium heat; add onions and saute for 2 minutes. Add pancetta (and mushrooms, if using); saute 2 minutes. Add spinach leaves; saute until wilted. Meanwhile, in a small bowl, whisk together the egg, egg white, Parmesan cheese, salt and pepper, to taste. Pour the mixture over the spinach/pancetta; cook until bottom has browned lightly. Place the skillet under the broiler (if using a skillet with a plastic handle, wrap the handle with aluminum foil prior to placing under the broiler); broil until the egg is set and lightly browned. Serve immediately.
Per Serving: 334 Calories (69.9% from fat); 26g Fat (8g Sat, 13g Mono, 3g Poly); 22g Protein; 3g Carbohydrate; 1g Dietary Fiber; 241mg Cholesterol; 714mg Sodium. Food Exchanges: 3 lean Meat; 1/2 Vegetable; 3 1/2 Fat.