Heat oven to 200C/180C fan/gas 6. Put the chipolatas in a baking dish approx 30 x 25cm (this will hold your pasta later, so make sure it’s big enough), drizzle with the tomato oil and cook for 25 mins.
Meanwhile, cook the macaroni in a large pan of boiling salted water until tender, then drain. While the pasta cooks, make the sauce. In a large pan, melt the butter, then add the spring onions. Fry for 1-2 mins to soften. Stir in the flour and mix with the butter to make a paste. Continue cooking for 30 secs, then add the milk, a little at a time, stirring continuously, until the sauce is thick and smooth. Add the cheddar and Parmesan (reserving a good handful for the top), half the semi-dried tomatoes, the rosemary, seasoning and macaroni, then stir everything together.
Remove the baking dish from the oven, transfer the sausages to a plate and pour away any oil from the dish. Tip in the macaroni, nestle the sausages back in, pushing them in until only half exposed, then sprinkle with the remaining cheese and tomatoes, and push in a few rosemary sprigs. Return to the oven and bake for 25 mins. Serve with a salad, if you like.