01 - Combine egg whites, salt, and black pepper in a medium mixing bowl. Whisk until mixture is frothy and well blended.
02 - Heat olive oil in a nonstick skillet over medium heat. Add red onion, bell pepper, and mushrooms. Sauté for 2 to 3 minutes until just tender.
03 - Incorporate cherry tomatoes and baby spinach into the skillet, stirring gently. Cook for 1 to 2 minutes until spinach wilts slightly.
04 - Pour whisked egg whites evenly over the sautéed vegetables. Reduce heat to low.
05 - Cover the skillet and allow the egg whites to cook undisturbed for 3 to 4 minutes until set.
06 - Sprinkle crumbled feta cheese over the surface if desired. Fold omelette in half using a spatula and transfer to a serving plate.
07 - Garnish with chopped fresh parsley and serve immediately.