01 - Whisk egg whites with salt and ground black pepper in a medium mixing bowl until frothy.
02 - Heat olive oil or coat a nonstick skillet with cooking spray over medium heat. Add red onion, diced bell pepper, and sliced mushrooms; sauté for 2 to 3 minutes until vegetables have softened.
03 - Incorporate halved cherry tomatoes and spinach leaves into the skillet. Cook for an additional 1 to 2 minutes until the spinach is just wilted.
04 - Pour whisked egg whites evenly over the sautéed vegetable mixture. Lower heat to a gentle setting.
05 - Cover the skillet and cook for 3 to 4 minutes, or until egg whites are fully set.
06 - If desired, sprinkle crumbled feta cheese across the surface. Carefully fold the omelette in half and transfer to a serving plate.
07 - Garnish with chopped fresh parsley and serve immediately.