1. Heat the 4 oil spoonfuls and the butter in a non-stick frying pen. Turn the onion golden, previously cut in thin slices, and add the onion. Put the lead on the pen and let it cook over a very low heat during 40 minutes, often stirring with a wooden spoonful. If necessary, add some spoonfuls of hot water.
2. Add the salt to the liver, flour it slightly and put it in the pen with the onions. Turn it brown uniformly and add the hot stock. Cook over a high flame during 5 minutes, without the lid.
3. In the meantime, fry the slices of white bread with the rest of oil. Put them aside in a platter and, in the centre, place the liver with the onion. Serve hot and add, if you wish, mashed potatoes.
Add a teaspoonful of sugar if you want to turn the onions golden faster.
You can use sherry, Oporto or brandy instead of the stock. In this case, you will obtain a liver much more spiced.