Heat oven to 200C/180C fan/gas 6. Toss the brussels sprouts in the oil and vinegar, then tip into a roasting tin. Bake for 10-15 mins, tossing again halfway through.
Meanwhile, cook the pasta until it has a slight bite (around two minutes less than pack instructions), then drain and set aside.
Heat the butter in a pan. Add the garlic and flour and cook for 1-2 mins. Gradually add the milk until the mixture is smooth, then simmer for 5 mins until thick.
Remove from the heat and add the mustard and most of the cheese, reserving one handful for the topping. Season well and stir. Add the pasta, then tip the mixture into a large baking dish. Top with the roasted sprouts and reserved cheese and bake for 20 mins until the cheese topping is golden and bubbling. Serve with a green salad, if you like.
