Portobello Pesto Egg Omelette

1 teaspoon olive oil

1 portobello mushroom cap, sliced

¼ cup chopped red onion

4 egg whites

1 teaspoon water

salt and ground black pepper to taste

¼ cup shredded low-fat mozzarella cheese

1 teaspoon prepared pesto

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.

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.