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.