Heat ½ teaspoon of the oil in a small non-stick saucepan over medium-high heat. Add the mushrooms and cook about 4 minutes or until softened. Add tomato sauce and simmer for 5 minutes or until thickened. Remove from heat, cover and keep warm.
Whisk egg whites, egg and pepper in a small bowl.
Heat the remaining oil in a large non-stick frying pan over medium heat. Add onion and cook until softened. Stir in spinach with salt and cook until heated through. Stir half of the spinach mixture into the egg mixture.
Coat the same frying pan with non-stick cooking spray. Heat over medium heat. Pour in spinach-egg mixture and spread evenly over pan. Cook without stirring until the eggs begin to thicken slightly around the edge.nute. Run a thin spatula around the edge of the pan, lifting mixture so the uncooked portion flows under the cooked one. Cook about f3 minutes or until centre is still moist but not runny.
Spread remaining spinach mixture over one half of the omelette and sprinkle with parmesan. Fold omelette over to cover filling. Reduce heat to low. Cover and cook until egg mixture is set but still soft - 3 to 4 minutes. Slide omelette onto serving plate and spoon over mushroom sauce.