Slice the liver very fine into pieces of about three inches in length and one in width. Season well with salt and pepper. Slice two onions very fine and take a tablespoonful of lard or butter and put into the frying pan. When it heats, add the onions, and, as they brown, place on top the slices of Liver. Let them brown on one side about two minutes and a half and then turn on the other. Let this side brown two minutes and a half longer and serve with the onion sauce, to which add a teaspoonful of vinegar.