Put the pig's liver slices in a shallow dish, cover with a little milk, and leave to soak for 30-60 minutes. Lamb's liver does not need soaking. De-rind the bacon. Peel and slice the onion into rings. Dry the liver on kitchen paper. Heat the oil and butter in a frying pan over a moderate heat. Add the onion slices and fry for 3-4 minutes, stirring frequently, until soft. Push the onion to one side of the pan and stir occasionally while frying the bacon and liver.
Put the bacon and liver in the hot fat in the pan, and fry gently for 3-5 minutes, turning frequently, until cooked to taste - liver should be soft on the outside, not crispy. Remove the liver, bacon and onion from the pan and serve on a hot plate.
Either pour the meat juices from the pan over the liver and serve with crusty new bread; or make gravy with the juices and serve with boiled potatoes and a green vegetable. For the gravy; mix the gravy granules (or flour and gravy flavouring powder) into a smooth paste with the sherry, wine, or cold water. Add the half cup of vegetable water or cold water and mix well. Pour onto the meat juices in the pan, stirring well. Bring to the boil, stirring all the time until the gravy thickens. Pour over the liver and vegetables.