Trim the cauliﬂower and steam or boil whole for 10–15 minutes or until the ﬂoret stems are only just tender.
Melt the butter in a saucepan on low heat then stir in the ﬂour and cook for 1 minute. Gradually stir in the milk and bring to the boil while stirring continuously. Reduce the heat and cook for 2–3 minutes more.
Add three quarters of the cheese and stir until the cheese melts. Remove the pan from the heat and keep hot.
When the cauliﬂower is just tender, drain well and put on a warm, heatproof serving dish. Pour the hot sauce over the cauliﬂower, sprinkle with the remaining cheese and serve at once. If you prefer, the top can be lightly browned by placing the dish under a hot grill for a few minutes.