Wash potatoes thoroughly and cook in boiling water for about 15 minutes. Drain, rinse and peel. Peel onion and dice very finely. Coarsely grate the potatoes. Season well with salt, pepper and nutmeg.
Heat the butter and oil in two portions in a frying pan (26 cm Ø), add half of the potato mixture to each portion and flatten to form a Rösti. Bake at low to medium heat for 4-5 minutes on each side until golden brown.
(Slide the Rösti onto a lid and turn carefully.) Slide out of the pan and keep warm.