Heat skillet to medium heat and add oil. Add in garlic, red cabbage, bell pepper, mushrooms, and grape tomatoes. Cook until cabbage and bell peppers are tender.
Toss in spinach and season with salt and pepper to taste. Remove from heat and transfer veggies to a small bowl. Cover to keep warm.
In a medium bowl, whisk the egg whites, water, and a pinch salt and pepper until frothy. Lightly coat a medium nonstick skillet with cooking spray and heat the skillet over medium heat.
Add half of the egg whites to skillet, swirling to evenly cover the bottom of the pan. Cook until set, about 1 1/2 to 2 minutes. Using a rubber scraper lift the eggs up and let the runny uncooked egg flow underneath.
Spoon half of the veggie mixture onto half of the omelet, sprinkle 1 tbsp of cheese (if desired), fold over, and slide onto a serving plate. Repeat with remaining egg whites and veggie mixture.
Nutritional Information (1 omelet): Calories 158, Carbohydrate 10g, Fat 7g, Protein 13g, Saturated Fat 1g, Fiber 4g, Sugars 5g