Cut the cauliflower into fourths. Discard the heavy stem and cut into florets. Simmer in plenty of boiling water for 6-8 minutes until soft and tender. Drain thoroughly.

In the meantime, heat the butter in a small saucepan. Stir in the flour and cook, stirring, 1 or 2 minutes. Add the milk gradually to the flour mixture. Stir with a wire whisk until thickened and bubbly. Season the sauce with the salt, pepper and nutmeg.

Pour the sauce into a blender or food processor. Add the cauliflower to the blender and puree the mixture until it is smooth. Serve immediately; or place in a greased casserole for later reheating.