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

* Remove the green outer leaves from the cauliflower and steam whole over a pan of boiling water for 10 minutes. Remove the cauliflower from the heat and leave to cool.

* 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 runny. Add the grated cheese and stir until melted. Remove from the heat.

* Break the cauliflower florets from the thick, central stalk taking care not to break it into tiny pieces. Place the florets in a baking dish large enough to hold all the florets in one layer.

* Pour the thickened cheeee 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.