01 - Whisk the egg whites with salt and black pepper in a medium bowl until frothy.
02 - Heat olive oil in a nonstick skillet over medium heat. Add red onion, bell pepper, and mushrooms. Sauté for 2 to 3 minutes until vegetables are softened.
03 - Stir in the cherry tomatoes and spinach leaves. Cook for 1 to 2 minutes until spinach has just wilted.
04 - Pour the egg whites evenly 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 fully set.
06 - If using, sprinkle feta cheese evenly over the surface. Fold the omelette in half and transfer carefully to a plate.
07 - Garnish with chopped fresh parsley and serve immediately.