Omelette with Balsamic Vinegar

For 4 persons. Cut the onion in very thin and little slices and put it in an anti-stick pan where you have already heated the olive oil. While the onion gets a bit brown, mix the eggs with the Parmesan cheese, the balsamic vinegar, the salt and the pepper. Put the mix in the pan and cook it on both sides. Let it get cold, slice it and serve it as hors d'oeuvre with a little spoon of balsamic vinegar on it (if you like it). It can also be eaten hot but in this case it is better presented as a second course.