Peel and wash the potatoes and cook them in boiling salted water for about 20 minutes. Roast mustard seed in a pan without fat for about 4 minutes. Wash or brush the black salsifies under running water, peel and place in vinegar water (3-4 tablespoons of vinegar and 1/2 litre of water).
Cut the black salsifies into pieces of about 5 cm length. Cook in boiling salted water with 4 tablespoons of milk for about 10 minutes. For the meatballs, peel 1 onion and cut into fine cubes. Knead minced meat, onion, egg, breadcrumbs, mustard and mustard seed.
Season with salt and pepper. Shape the mince into 8 meatballs. Heat oil in a pan. Fry the meatballs in portions for about 8 minutes while turning. Peel 2 onions and cut into rings. Dust with flour and season with salt.
Bring cream and 100 ml milk to the boil. Pour black salsify into a sieve, drain and simmer in the boiling cream for about 5 minutes. Season with salt and pepper. Remove the meatballs and keep them warm.
Fry onion rings in hot frying fat for about 3 minutes until crispy. Bring 250 ml milk and butter to the boil. Drain potatoes. Add the butter-milk mixture to the potatoes and mash the potatoes into puree. Flavour with nutmeg.