Peel and slice the onions.
Add the oil (a fruity olive oil works well too) to a shallow, hot pan and fry the onions until the are sweet.. They should be soft, but not crispy, so put a lid on so they steam as well as fry for around 8-10 minutes.
Cook the pasta in boiling salted water according to packet instructions.
When the onions have caramelised, skin the sausages and cut the meat into bite-size chunks (usually about 3 or 4 per sausage). Add them to the onions and cook for a few minutes.
Chop and add most of the flat-leaf parsley (about a handful). Leave a little left for garnish at the end.
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 if usually enough.
Add the double cream and stir to combine. Check the seasoning. Add more mustard if you want.
Drain the pasta and add to the pan to combine. Add a tablespoon of pasta water if it’s too thick or you want a more liquid sauce.
Serve immediately with an extra sprinkling of chopped parsley.