Preheat the oven to 180 degrees C
Steam the cauliflower for about 10 minutes, until slightly soft but not soggy
Meanwhile, place the milk in a heavy bottomed saucepan
Add the flour and butter
Slowly bring to the boil stirring continuously with a balloon whisk
Simmer gently for a few minutes
Stir in 3/4 of the cheese and allow to melt
Stir in the mustard
Once the cauliflower is cooked, turn into a casserole or baking dish
Pour the cheese sauce over the top and stir to cover all the cauliflower with sauce
Sprinkle over the remaining cheese
Bake in the oven for about 30 minutes, until the top is golden brown
Serve with your favourite green vegetable