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Cook the macaroni following pack instructions, adding the cauliflower for the final 4 mins.
Melt the butter in a pan, then stir in the flour and mustard powder and cook for 2 mins. Gradually add the milk, stirring all the time to get a smooth sauce. Add three-quarters of the cheese and some seasoning to the sauce.
Drain the macaroni and cauliflower and stir into the cheese sauce. Transfer to an ovenproof dish, then sprinkle over the remaining cheese and flash under a hot grill until golden and bubbling. Serve with a green salad, if you like.