In a large nonstick frying pan, melt the butter over medium heat. Add the onion and cook until golden brown, about 4 to 5 minutes. Add the mushrooms and vinegar and cook until the mushrooms are tender, around 3 minutes. Transfer to a small bowl.

In a large bowl, combine the egg whites, milk, salt and pepper to taste and whisk until well mixed.

Generously coat a nonstick omelette-size frying pan with cooking spray and heat over medium heat. Place egg white mixture in pan and cook for 1 minute, then spread the onion mixture over half of the eggs. Cover, reduce heat to medium-low and cook until the eggs are cooked through and the center of the omelette is set, 4 to 5 minutes. Using a wide spatula, loosen the underside of each omelet that doesn’t have the onion mixture on it, fold in half to enclose the onion mixture, cover and cook for 1 minute more.

Slide the omelette onto a plate and garnish with chopped parsley. If making more than two, place cooked omelettes in a covered baking dish at 250 degrees to keep warm.