1. Heat the fat or drippings in a large frying pan and saute the sliced onions until they are tender and yellow, 5 to 10 minutes. Remove from the pan and keep them hot.
2. Have the liver sliced in uniformly thick slices, 1/2 inch or slightly less. Remove the membrane and cut out large tubes. Wipe with a clean, damp cloth. Dip into flour.
3. Brown slices on both sides in the fat left in the pan. Cook about 3 minutes on each side. Place on a warmed platter, sprinkle with salt and pepper. Surround with onions, or spread the onions on top of the slices, before adding salt and pepper.