Preheat the oven to 200c.
Place the cauliflower in a large saucepan of salted water, bring to the boil and cook for 5 minutes, tip into a colander and leave to drain.
To make the sauce melt the butter in a saucepan and stir in the flour, cook over a gentle heat for 1 minute. Remove from the heat and add the milk a little at a time, stirring well between each addition. Return the pan to a medium heat and bring to the boil stirring constantly, then simmer for 2 minutes. Remove from the heat and add the mustard and 2/3 of the cheese.
Place the cauliflower in an ovenproof dish, pour over the sauce making sure to cover all the cauliflower, then scatter over the remaining cheese, season with salt and pepper.
Bake in the oven for 30 minutes or until the top is golden brown.