Peel the onions and thinly slice them. Sauté them in extra virgin olive oil for a few minutes, letting them soften and turn golden; then transfer them to a bowl to let them cool. Add the 6 eggs to the onions and beat with a whisk or fork until the mixture is smooth and airy. In a copper frying pan, heat about 2 tablespoons of oil, then pour in the egg mixture. When the frittata no longer sticks to the bottom, it is ready to be flipped over to cook the other side. To finish, transfer the omelette to a serving dish and drizzle it with Traditional “Extra Vecchio” Balsamic Vinegar of Modena PDO at the table.