Trim and prepare the cauliflower and cut into florets - or leave whole (after removing the centre stalk core and cutting across in what core is left). Add to boiling water and cook until nearly ready but still firm.
Drain thoroughly and put into a warm heatproof dish
Grate the cheese. Melt the margarine and stir in the flour to make a roux sauce (cooking until sandy in colour and texture). Add the milk, stirring all the time, and cook until thickened
Stir in 3/4 of the cheese and all of the mustard. Place the cauliflower in a serving dish and pour over the cheese sauce.
Sprinkle with the remaining cheese and put in a moderate oven until bubbling and golden.