Preheat the oven to 200C/180C fan/Gas 6. Bring a large pan of water to the boil.
Boil the cauliflower florets for 3-4 minutes until just tender but still with some bite. Drain well and tip into a baking dish.
Make the sauce: melt the butter in a saucepan, stir in the flour and cook for 1 minute to make a paste.
Add the milk a splash at a time, stirring well after each addition, until you have a smooth sauce. Then pour in the rest and simmer for 2-3 minutes until it thickens.
Stir in three quarters of the cheese, the mustard, salt and pepper until the cheese has melted.
Pour the sauce evenly over the cauliflower and scatter the remaining cheese on top.
Bake for 15-20 minutes until golden and bubbling at the edges. Serve with jacket potatoes or crusty bread.