1. Cook the pasta according to pack instructions. Drain thoroughly.
2. Meanwhile, make the sauce. Melt the butter in a pan and stir in the flour and mustard. Cook, stirring, for 30 seconds, then slowly add the milk to give a thin smooth sauce. Simmer for 5 minutes, stirring occasionally, then stir in the cheese until melted.
3. Mix in the pasta and either serve straight away or transfer to a heatproof dish and top with extra grated cheese. Bubble under the grill, then serve with grilled tomato halves or a tomato salad.