Peel and finely chop the onions. Add to the hot butter in a frying pan and cook gently for about 5 minutes until the onion is tender. Meanwhile trim the liver and cut each slice across into thin strips rather as you would cut steak for beef stroganoff. Toss the strips of-liver in seasoned flour and add to the pan of onions. Fry both together over fairly high heat, tossing the ingredients to seal the liver on all sides. Allow to cook for about 4–5 minutes, then with a perforated spoon lift the liver and onions out on to a hot serving dish. Add the wine to the hot pan and allow it to boil up fairly fast. Stir to pick up all the flavouring bits in the pan and let the wine reduce by about half to concentrate the flavour. Pour over the liver and serve.