Heat the butter and oil in a large pan and fry the liver for 3 - 4 minutes on each side. At the same time, brown the onions on the edge of the pan. When the underside of the liver is browned, turn and season with salt.
Serve with the browned onion rings.
Serve with mashed potatoes.
Tip: Pork and beef liver will be more tender if you soak them in milk for 30 minutes before roasting.