01 - Add egg whites, salt, and black pepper to a medium mixing bowl. Whisk vigorously until the mixture becomes frothy.
02 - Heat olive oil or nonstick cooking spray in a nonstick skillet over medium heat. Add red onion, bell pepper, and mushrooms. Cook for 2 to 3 minutes, stirring occasionally, until vegetables are softened.
03 - Add cherry tomatoes and baby spinach leaves to the skillet. Sauté for an additional 1 to 2 minutes until spinach just wilts and tomatoes soften.
04 - Pour the prepared egg whites evenly over the sautéed vegetables in the skillet. Reduce heat to low.
05 - Cover the skillet and cook for 3 to 4 minutes until the egg whites are fully set.
06 - If using, sprinkle crumbled feta cheese over the surface. Gently fold the omelette in half using a spatula and slide onto a serving plate.
07 - Garnish with chopped fresh parsley and serve immediately.