Cut the cauliflower into medium sized florets, place in a large pan and add a few cm (1inch) boiling water and ½ teaspoon salt. Put the lid on the pan, return the water to the boil and steam the cauliflower for 5 minutes until beginning to soften.
Drain, and leave uncovered to allow some of the steam to evaporate.
Place 350ml (1½ C) of the milk and the cheddar, mustard, salt and pepper into a medium sized pan. Place over a medium heat and allow to warm.
Stir the corn flour and the remaining milk together and stir into the warming cheese sauce. Continue to heat, stirring constantly (use a whisk if you wish) until the sauce thickens. Continue to stir, cooking for a further two minutes.
Place the drained cauliflower into an oven proof dish. Pour over the cheese sauce and place in the pre-heated oven for around 15 minutes until bubbling and golden.