This recipe is so adaptable, so change the mustard and the sausage to suit your own taste; for example, if you like spicy sausages why not marry them with sweet mustard?
Add the oil to a hot, shallow, large pan and fry the onions until they are very, very sweet. They want to be soft, not crisp, so put the lid on so they steam as well as fry, say a good 8-10 minutes. Meanwhile cook the pasta in boiling salted water according to packet instructions.
When the onions are caramelised, skin the sausages and rip the meat into good sized chunks and add to the onions and cook for a few minutes.
Add in the flatleaf parsley and stir in the mustards. It's up to you to add as much or as little as you like of each. A tablespoon of each is usually enough.
Stir in the cream and check the seasoning, adding more mustard if you want. Drain the pasta and add to the cream and stir well. Serve immediately with an extra sprinkling of chopped parsley.
