01 - In a medium bowl, whisk egg whites with salt and black pepper until foamy.
02 - Heat olive oil in a nonstick skillet over medium heat. Add red onion, bell pepper, and mushrooms, and sauté for 2 to 3 minutes until the vegetables soften.
03 - Incorporate cherry tomatoes and spinach into the skillet and cook for 1 to 2 minutes until spinach just wilts.
04 - Evenly distribute the prepared egg whites over the sautéed vegetables in the skillet. Reduce heat to low.
05 - Cover and allow to cook for 3 to 4 minutes, or until egg whites are completely set.
06 - If using, sprinkle feta cheese over the omelette. Fold in half and slide onto a plate.
07 - Garnish with fresh parsley and serve immediately.