Peel, halve and finely slice the onions.
Put the onions into a pan with the olive oil, butter and a pinch of salt. Cook over a low heat for 45 minutes to 1 hour, stirring every now and then, until golden and caramelised.
Meanwhile, cook the sausages. Either bake them in the oven at 180°C for 25 minutes, or fry them for about 10 minutes, until cooked through. Slice into bite-sized pieces.
Cook the pasta according to packet instructions, then drain.
When the onions are caramelised, add the sliced sausages, 50ml water and double cream to the pan. Stir well.
Tip in the drained pasta and stir again until coated in the sauce.
Taste and add salt and pepper as needed before serving.