Peel, wash and dice the potatoes and cook in boiling salted water for about 15 minutes. In the meantime clean, wash and cut leek into rings. Peel the skin off the sausage. Cut the sausage into slices about 1/2 cm thick.
Heat 40 g butter in a frying pan. Brown the sausage slices on both sides. Remove from the pan. Brown the leek and dust with flour. Add vegetable stock and 200 ml milk, bring to the boil.
Season with horseradish, salt and pepper. Stir in slices of sausage. Drain the potatoes. Add 150 ml milk and 1 tablespoon butter and mash to a puree. Season with salt and nutmeg. Grease casserole dish.
Pour in the puree in strips. Fill the sausage ragout into the spaces between the strips. Melt 1 tablespoon of butter and pour over the puree. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas mark 3) for approx. 25 minutes.