01 - Combine egg whites with salt and black pepper in a mixing bowl and whisk until frothy.
02 - Heat olive oil in a nonstick skillet over medium heat. Add red onion, diced bell pepper, and sliced mushrooms; sauté 2 to 3 minutes until vegetables are softened.
03 - Incorporate halved cherry tomatoes and baby spinach leaves. Cook for 1 to 2 minutes until spinach wilts.
04 - Pour whipped egg whites evenly over the sautéed vegetables. Reduce heat to low.
05 - Cover skillet and cook for 3 to 4 minutes until egg whites are just set.
06 - Optional: sprinkle crumbled feta cheese over the omelette. Fold in half, transfer to plate, and garnish with fresh parsley. Serve immediately.