1) In a large omelet pan, saute the onions and parsely in the butter, until brown.
2) Season with the salt and pepper to taste.
3) Pour the eggs over the onion mixture and cook, until the omelet is brown underneath and is almost set on top.
4) Turn over and cook for further 1 minute and remove on a warmed serving dish.
5) In the hot pan, bring the vinegar to a boil and drizzle over the omelet just before serving.
6) Slice and serve the Lyons Onion Omelet on individual serving plates.