Slit open the sausages and remove the meat, discarding the skins.
Heat the olive oil and butter in a frying pan and cook the onion gently for 5-6 minutes until soft but not coloured.
Pinch the meat into the pan and fry, breaking it up with a wooden spoon until it is cooked but not browned. Add the milk and simmer very gently, stirring, for 5 minutes until it is all absorbed.
Add the tomatoes and their juice to the pan with the sugar, nutmeg and salt and pepper. Simmer for 15 minutes, stirring now and then.
Meanwhile cook the pasta in a large pot of boiling salted water until al dente.
Add the cream to the sauce if using and heat through gently.
Drain the pasta, toss well with the sauce and serve with plenty of freshly grated parmesan.