Put the onion and mussels together in a pan and fry them. When the onion turns golden yellow, deglaze with white wine. Let simmer further. As soon as the wine has been reduced, add the stock and turn the temperature back down. Continue simmering until the mussels are through.
Finally season with salt, pepper and nutmeg and refine with parsley and crème fraiche.
Arrange the pasta with the sauce on plates and serve immediately.