Boil the potatoes with the skin on or steam them after peeling and quartering them. Meanwhile, sauté the crumbled sausage in a bit of butter.
Mash the potatoes and mix in the sausage, remaining butter (except for a small piece), egg, cream, and Parmigiano. Season with salt, pepper, and nutmeg.
Transfer the mixture to a buttered and breadcrumb-coated mold. Bake for about 45 minutes and serve hot.