Heat the oven to 200C/395F/Gas 7

Remove the green outer leaves from the cauliflower and break the cauliflower florets from the thick, central stalk taking care not to break them into tiny pieces.

Place the florets in a greased baking dish large enough to hold all the florets in one layer.

Place the butter and flour into a large saucepan. Over a low heat stir the butter and flour until the butter has melted and the flour is incorporated. Add the salt and mustard powder and continue stirring for 2 minutes.

Turn the heat up to medium and add the milk in one go and whisk furiously until all a smooth sauce is formed. Continue stirring until the sauce is thickened and glossy (about 5 minutes) If the sauce is very thick add a little more milk, the sauce should be thick but still a little on the runny side. Add the grated cheese and stir until melted. Remove from the heat.

Pour the thickened cheese sauce over the cauliflower ensuring all the florets are covered. Sprinkle with grated cheese and a good twist of black pepper.

Bake in the hot oven until the sauce is bubbling and golden brown on the top, approx 30 minutes.