Wash the potatoes and cook in boiling water for about 20 minutes. Drain, rinse with cold water and peel. In the meantime, clean and wash the leek and cut into rings. Cook in 200 ml boiling salted water for about 5 minutes. Drain while collecting the stock.
Cut the sausages into slices. Peel and finely chop the onion. Heat the fat. Sauté diced onion in it. Dust with flour and sweat. Deglaze with leek stock, whipped cream and milk. Bring to the boil while stirring constantly. Season to taste with salt, pepper and mustard. Warm up the potatoes, leek and slices of sausage.
Dust with flour and sweat. Deglaze with leek stock, whipped cream and milk. Bring to the boil while stirring constantly. Season to taste with salt, pepper and mustard. Warm up the potatoes, leek and slices of sausage. Wash parsley, except for a little bit for garnishing, chop and sprinkle over it