Heat the oil in a frying pan. Fry the liver in batches for approx. 1 min. per batch, remove from the pan, keep warm.
Reduce the heat, sauté the onions in the same pan for approx. 2 mins. Pour in the wine and stock, simmer the onions for approx. 10 mins. until soft. Season the liver, add the parsley and butter, serve immediately.