Peel, wash and chop the potatoes. Cook in salted water for about 20 minutes
Clean, wash and cut the leek into rings. Peel the skin off the sausage. Cut the sausage into slices about 1/2 cm thick. Brown in 1 tbsp. hot butter, take out
Heat 1 tablespoon of butter in the frying fat. Sauté the leek in it. Stir in 200 ml milk, 300 ml water and stock. Bring to the boil and simmer for about 10 minutes. Bind with sauce thickener. Season to taste with horseradish, salt and pepper. Fold in slices of sausage
Drain the potatoes. Add 150 ml milk and 1 tablespoon butter, mash. Season to taste with salt and nutmeg. Grease an oven dish (approx. 30 cm long). Spread 1/3 puree into it. Fill sausage ragout on top. Spread the rest of the puree in thick strips. Spread 1 tablespoon of butter in flakes on the puree. Bake in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas mark 3) for approx. 25 minutes