01 - Whisk egg whites in a medium mixing bowl with salt and black pepper until frothy.
02 - Heat olive oil or nonstick cooking spray in a nonstick skillet over medium heat. Add diced red onion, bell pepper, and sliced mushrooms; sauté for 2 to 3 minutes until vegetables are tender.
03 - Incorporate cherry tomatoes and spinach leaves into skillet; cook for 1 to 2 minutes until spinach is wilted.
04 - Pour whisked egg whites evenly over sautéed vegetables in the skillet. Reduce heat to low.
05 - Cover with lid and cook for 3 to 4 minutes until egg whites are fully set.
06 - If desired, evenly sprinkle crumbled feta cheese over the set egg whites. Using a spatula, fold the omelette in half and transfer gently to a plate.
07 - Garnish omelette with freshly chopped parsley and serve immediately.