Peel the apple and cut into wedges. Caramelize with 1 tbsp. butter and sugar in a non-stick frying pan. Remove all tendons and membranes from the duck liver and cut into portion pieces. Clean, rinse and spin dry the leaf lettuce. Stir a salad dressing from sunflower oil or other vegetable oil, walnut oil, sherry vinegar, salt, sugar and pepper and mix with the lettuce leaves. Arrange the leaf lettuce on 4 plates.
Roast the duck liver in butter on both sides in a coated frying pan until golden brown. Only at the end season with salt and pepper and extinguish the roast stock with balsamic vinegar and port wine.
Serve together with the warm apple slices on the leaf salad and sprinkle with walnut pieces.