You need a grill for this. It takes half an hour. I think it's better with broccoli.
Chop away the rough parts of the cauliflour or broccoli, and divide what remains into florets about an inch in size. Boil enough water to cover them (a bit less, actually) in a saucepan, add some salt, and add the florets. Cover and simmer for about five minutes. You should be able to pierce the stemmy parts easily with a fork. Drain and set aside.
Melt the butter in a small pan over low heat. Add the flour and mix it in. You should have a greasy mass in the bottom of the pan, with the consistency of toothpaste; adjust by adding flour or butter. There should be enough to fill a serving spoon. Heat that mass for a minute or two, stirring. Don't let it burn.
Turn off the heat, and add a splash of the milk. Stir vigourously until the mixture is smooth. Keep adding milk in splashes, and stirring it in, until the mixture has the consistency of single cream. If it gets lumpy (and you can't seem to eliminate the lumps by stirring), press it through a sieve or whizz it with a blender. There should be no lumps.
Add a couple of handfuls of grated cheese, and turn the heat back on. Add enough mustard to cover the tip of a teaspoon. Heat and stir until the cheese has melted. Stir in more milk until the mixture again has the consistency of single cream. Add salt and pepper, and check the flavour.
Put the cooked florets in an ovenproof dish. Pour over the cheese sauce. Grate some more cheese over the top. Grill until brown (about 5 minutes).