I love cauliflower and I love it done like this. It is very simple. The trick of course is to cook the cauliflower to the point where it will hold together and still be cooked throughout. The takes a bit of practice but a general rule is to cook it in plain boiling water with a little salt until just you can break off one of the buds. Then you plunge it into iced water to stop it from cooking. You keep the original cooking water warm and when you are ready to dress your cauliflower you heat it up in there.
Mornay sauce is just bechamel with Swiss cheese and a beaten egg yolk cooked over hot water as you would for a hollandaise sauce. You will find perfectly good recipes for this on the Internet but one little tip for your bechamel : add a bay leaf, a couple of cloves and some onion to it while it is cooking and make sure that it cooks for a good ten minutes on a slow heat.