Wash the liver very briefly under water and dry with a kitchen towel.
For the appetizer, remove the skin from the apples and cut out the core with an apple corer.
Cut the apples into thick slices and drizzle with the juice of one lemon to prevent them from discoloring.
Peel the onions and cut them into narrow rings.
Heat two frying pans, in one frying pan heat the butter, brown the onion rings in it, remove and fry the apple slices for 3 to 4 minutes.
In the second frying pan, heat the oil. Dredge the liver in flour on both sides, pressing the flour well with your hands until smooth.
Fry the liver slices in the hot fat at a low temperature for 3 minutes on both sides, then add salt and pepper.
Arrange the fried liver slices on a preheated plate and surround with apple slices and onion rings.