Bring a large pan of salted water to the boil, add the cauliflower and cook for 15mins, or until tender. When cooked, drain and keep to one side.
Preheat the grill to high.
In a large saucepan melt the butter, whisk in the flour and cook for 1-2 mins.
Whisk in the milk slowly until you have a thick sauce, then add the mustard and half the grated cheese and whisk until smooth. Season with salt and pepper.
Add the cauliflower to an ovenproof dish and pour the sauce over until covered.
Sprinkle the remaining cheese on top.
Place under the grill for 5mins or until golden and bubbling.
