1.Heat The Olive Oil In A Skillet Over Medium Heat. Cook The Portobello Mushroom And Red Onion In The Olive Oil Until The Mushroom Has Softened, 3 To 5 Minutes. 2.Whisk The Egg Whites And Water Together In A Small Bowl; Pour Over The Mushroom And Onion Mixture. Season The Egg Whites With Salt And Pepper. Cook, Stirring Occasionally, Until The Egg Whites Are No Longer Runny, About 5 Minutes. Sprinkle The Mozzarella Cheese Over The Mixture; Top With The Pesto. Fold The Omelette In Half And Continue Cooking Until The Cheese Melts, 2 To 3 Minutes.