1. Peel and cut the onions in half and slice really thinly.
2. In a large frying pan, heat the olive oil, put in the onions and stir. Cook them for 15 minutes until they are golden brown.
3. Season if necessary, remove the onions from the frying pan and keep warm.
4. Now fry the liver. Put the slices into the frying pan in a single layer so they do not overlap and add the vinegar. Allow no more than 30 seconds each side.
5. if you cook it really fast it stays just that tiny bit pink in the middle, and delicate and delicious.
6. As soon as the second side has had its 30 seconds, take the liver out and put it on top of the onions.
7. Pour the pan juices, which are now sweet and sour from the aromatic caramelisation of the onions and the distillation of the vinegar, over the lot. and Serve