01 - In a medium mixing bowl, vigorously whisk the egg whites with salt and black pepper until frothy.
02 - Heat olive oil in a nonstick skillet over medium heat. Add red onion, bell pepper, and mushrooms, sautéing for 2 to 3 minutes until tender.
03 - Stir in cherry tomatoes and baby spinach leaves; cook for another 1 to 2 minutes until spinach wilts.
04 - Evenly pour the whisked egg whites over the sautéed vegetables in the skillet. Reduce heat to low.
05 - Cover the skillet and cook for 3 to 4 minutes, or until egg whites are just set.
06 - If desired, sprinkle feta cheese over the surface. Fold the omelette in half and transfer to a serving plate.
07 - Garnish with chopped fresh parsley and serve immediately.