Heat a non-stick skillet over medium heat and add olive oil or spray with cooking spray.
Sauté bell peppers and mushrooms until tender, about 3-4 minutes.
Add the spinach and cook for another 1-2 minutes until wilted.
Pour in the egg whites and season with salt and pepper.
Cover the skillet with a lid and cook until the egg whites are set, about 2-3 minutes.
Sprinkle cheese on one half and fold the omelette over.
Slide onto a plate and serve warm.