In a large nonstick frying pan over medium heat, heat the margarine. Add the onion and sugar and cook until the onion is tender and golden brown, about 4 minutes. Add the mushrooms and vinegar and cook until the mushrooms are tender, about 3 minutes. Transfer the onion mixture to a small bowl.

In a large bowl, combine the egg whites, milk, salt and pepper. Using an electric mixer or by hand, beat until stiff peaks form.

Wipe out the frying pan and coat with nonstick cooking spray. Place over medium heat; add the egg-white mixture and gently flatten it using the back of a spoon. Spread the onion mixture over half of the eggs. Cover, reduce the heat to medium-low and cook until the eggs are cooked through and the center of the omelette is set, 4-5 minutes. Using a spatula, carefully loosen the underside of the omelete, fold in half, cover and cook for 1 minute more.

To serve, slide the omelette onto a plate and garnish with the parsley. If preparing several omelettes, keep the first ones warm, covered with foil, in a 250 oven until all are cooked.