Boil the potatoes until soft. Soak the porcini mushrooms in warm water. Dice the bacon and fry in a pan until translucent, add the sauerkraut and a little water and simmer for 10 minutes. Strain the porcini mushrooms, chop and add to the sauerkraut with the crème fraîche. Sauté for a further 10 minutes, stirring constantly. Remove from the heat and keep warm. Peel and grate the onion. Peel the potatoes and press through a potato ricer. Mix the onion, herbs and warm milk into the mixture. Season with salt, pepper and nutmeg. Heat the oil in a pan. Form 4 balls from the potato mixture, place in the pan, press flat and fry on both sides.
Top with the sauerkraut mixture and serve.