Grate the potatoes coarsely into the a clean tea towel. Fold the towel around the potato to form a ball and squeeze to remove as much moisture as possible. Season the potato with black pepper then divide into six equal portions.
Heat a large frying pan over a medium heat and add the duck fat and the sunflower oil. Place a metal chef's ring inside the frying pan carefully fill with the one portion of grated potato. Using the back of a spoon gently push down to make a compact cake. Remove the ring and repeat with the remaining potato until you have six röstis.
Fry the röstis for 3-4 minutes on both sides, or until golden-brown , adding more oil or fat if required. Season with salt, then remove from the pan and drain on kitchen paper. Place onto a roasting tray and reheat before serving.
Remove the röstis from the roasting tray and serve onto your plate and spoon over an accompaniment such as meatballs
«You can use clarified butter if you are a vegetarian.»