01 - In a medium mixing bowl, combine the egg whites with salt and black pepper. Whisk thoroughly 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 - Incorporate cherry tomatoes and spinach leaves into the skillet. Cook for an additional 1 to 2 minutes, stirring gently, until spinach is just wilted.
04 - Pour the prepared egg whites evenly over the vegetables in the skillet. Reduce the heat to low for gentle cooking.
05 - Cover the skillet and cook for 3 to 4 minutes, allowing the egg whites to set without browning.
06 - Scatter crumbled feta cheese over the surface, if using. Using a spatula, fold the omelette in half and transfer to a plate.
07 - Sprinkle chopped fresh parsley on top and serve immediately.