Section the cauliflower and put it into a roomy pot of boiling water, and add a little bit of salt. Boil for 15-20 minutes or until tender. While the cauliflower is boiling, put a big knob of butter into a medium-sized saucepan, and let it melt. Add in the flour, a bit at a time, stirring it while doing so. When you've made a paste, start to add the milk, again, bit by bit. Stir continously, until the sauce has thickened, and can cover the back of a spoon. At this point, you can add the cheese. Mix until the cheese has melted into the sauce, and add the mustard. Season with salt and pepper to taste.
