01 - In a medium mixing bowl, vigorously whisk egg whites with salt and ground black pepper until frothy.
02 - Heat olive oil or apply nonstick cooking spray to a nonstick skillet over medium heat. Add chopped red onion, diced bell pepper, and sliced mushrooms. Sauté for 2 to 3 minutes, stirring occasionally, until vegetables are softened.
03 - Stir in the halved cherry tomatoes and baby spinach leaves; cook for an additional 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 skillet and cook for 3 to 4 minutes, or until the egg whites are just set.
06 - If using, sprinkle crumbled feta cheese over the omelette. Gently fold omelette in half and slide onto a plate.
07 - Garnish with chopped fresh parsley and serve immediately.