Grease a 24cm round baking dish with butter and set aside.
Roll the sausage meat into balls about the size of large cherries. Warm the olive oil in a large frying pan over a medium heat and fry the meatballs until golden on all sides. Remove and set aside.
Add the onion to the pan and cook for 7–8 minutes or until softened. Pour in the wine, scraping the bottom of the pan to lift any brown bits stuck to the bottom. Simmer for 2–3 minutes, then add the cream, nutmeg, sage and porcini. Cook for a further 2–3 minutes or until the porcini has softened and the sauce is slightly reduced. Add the meatballs to the sauce and keep warm.
Preheat the oven to 190C (210C conventional).
Cook the rigatoni in a large pot of salted water until just before al dente. Drain and toss through the sauce, along with the fontina and most of the 50g parmigiano reggiano. Season to taste, then transfer to the prepared baking dish.
Top with the rest of the parmigiano reggiano and bake in the preheated oven for 25–30 minutes or until bubbling and golden. Serve with some dressed salad leaves and extra parmigiano reggiano.
