In a large frying pan (with a lid), on medium heat, add olive oil and a knob of butter butter, then add the onions and garlic.Season, stir, then turn the heat down and pop a lid on and cook for 35. -45 mins, stirring regularly.Remove the lid and turn the heat up a little for 5 mins to get a bit more colour on them before setting to one side.
Using the same pan, add a little oil and the sausages. Cook until browned all over - approx. 10 mins. Once done, remove from the pan and let cool before you cut them into 2 cm slices.
While the sausages cool, cook the pasta according to the packet's instructions.Drain the pasta water but keep about 1/2 cup to the side.
Add the onions and sliced sausages back into the frying pan on low heat. Add in the wholegrain mustard and cream, season with pepper, and stir in. Then add the pasta water to make a nice creamy sauce.
Add them back to the pan along with the onions. Return the pan to the heat. Now add the cream and mustard and enough pasta cooking water to give the sauce a nice loose consistency. Add the pasta and parsley. Mix together and serve with some more fresh parsley to garnish.