Remove fat, tendons and skins from the liver and turn in flour. Season with pepper. Cut the apple and onion into slices.
Fry the liver in butter on both sides for 1 to 2 minutes. Remove from the pan and keep warm in the oven at approx. 100 degrees. Sauté apples and onions in the same pan. If necessary, add a little more butter and sugar to caramelize.
Season the liver with salt and serve with apples and onions.