Peel and slice the onions thinly; fry them in extra virgin olive oil, allowing them to soften and brown for a few minutes; then transfer them to a bowl to cool. Add the 6 eggs to the onions and beat with a whisk or fork until the mixture is smooth and fluffy. Heat the oil (about 2 tablespoons) in a copper pan, then pour in the mixture to cook. When it comes away from the bottom of the pan, it is ready to be turned over to finish cooking. To finish, transfer the omelette to a serving dish and pour over the Extra Aged Traditional Balsamic Vinegar of Modena P.D.O. in front of your guests.