Start by cooking the onions. Place a large frying or saucepan over a medium heat. Add the olive oil, followed by the butter, when it’s hot and bubbling away, add the onions and garlic to the pan. Break them up with a wooden spoon. Add a little salt and pepper and give them a good stir.
Turn the heat down, set a lid over the pan and cook the onions stirring regularly for 35 – 40 minutes. Now take the lid of the pan, turn up the heat a tad and get a little colour on them, if they aren’t already looking caramelised and delicious.
Scoop the onions out of the pan and set them to one side. Return the pan to the heat and add the remaining oil. Add the sausages to the pan and fry them for 12 – 15 minutes on all sides. Take the pan off the heat, lift them out of the pan and allow them to cool for a moment or two.
Meanwhile, cook your pasta according to the packet's instructions, or to your own liking. Drain the pasta but keep some of the cooking water.
Slice the sausages into 2 cm rounds. 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 and season well with sea salt and black pepper. Tumble the pasta round the pan until it’s hot and saucy, adding more pasta cooking water if you need to.
Tip the pasta out onto a serving dish or simply bring the pan to the table.