Peel the potatoes and garlic cloves. Boil and mash to a smooth puree with the milk. Beat in the cheeses and keep the fonduta warm by standing the pan in a roasting tin of hot water on top of the stove on a very low gas.
Meanwhile heat the oil and butter in a heavy frying pan. Dip the liver in the seasoned flour and fry for a few minutes on each side until done to your liking.