Only true gourmets have the heart to mash potatoes — but the heart, or rather the stomach, wants what it wants.
Coarsely slice the sausage. Slice the leek and dice the apple. Put the sausage on a pan and fry it until it turns golden brown. Add the leek and the apple, and fry it all until the leek is soft and the apple is golden brown. Season with marjoram, salt, and pepper. Put it all into a baking dish.
Press the boiled potatoes or mash them thoroughly. Use a saucepan to heat the butter and milk, and pour hot over the potatoes. Mix everything up. Add grated Parmesan cheese and season with salt, pepper, and nutmeg.
Put the prepared potatoes on the leek and sausage. Bake in the oven preheated to 200 °C (392 °F) for about 20 minutes, or until the potatoes are golden brown.