1 medium green bell pepper, seeded, cut in thin strips
Salt, as desired
3 large egg whites
1 large egg
1 tsp. grated Parmeggiano-Reggiano cheese
In a small saucepan, heat marinara sauce, vinegar and soy sauce, just until hot. Season with pepper to taste and set aside.
Coat medium non-stick pan liberally with cooking oil spray. Set pan over medium-high heat. Sauté onion until lightly colored, about 3 minutes. Add mushrooms, pepper and a pinch of salt. Cook, stirring often, until mushrooms are soft, about 4 minutes. Transfer to a plate. Wipe out pan.
Whisk egg whites and egg with cheese, a pinch of salt and a few grinds of pepper. Coat pan with oil spray and heat over medium-high heat until hot. Add egg mixture, tilting pan to coat bottom evenly. Reduce heat to medium. Cook until eggs are well-set, about 4 minutes. Slip spatula gently around edges, then underneath to loosen omelet. Slide it onto a serving plate.
Spoon filling to cover half of omelet and fold omelet, covering filling. With a spoon, drizzle sauce in a ribbon effect across omelet. Serve immediately.