The best way to do this is to heat the groundnut oil in a deep-fryer (to180˚C/350˚F) and melt the butter and oil for the liver in a frying pan simultaneously, so that you can cook both the liver and the onions at more or less the same time.
Also have some serving dishes keeping warm in the oven with some kitchen paper on them to absorb any excess oil. The liver slices should be coated in seasoned flour and fried quickly over a high heat in the butter and oil (about 1 minute on each side), then kept warm. The onions should be separated into thin rings, dipped first in plain flour, then in stiffly beaten egg white, and deep-fried till golden and crisp (for 1 or 2 minutes – it’s best to do them a few at a time).
Finally sprinkle 1 level tablespoon of plain flour into the pan juices left from the liver and cook it for a minute or two, then gradually stir in the stock to make a gravy, seasoning well. Serve the liver garnished with onions, and the gravy separately.