Preheat the oven to 210°C.
Break or chop the cauliflower into florets and then rinse under cold water. Place the milk, a jugful of water and the cauliflower in a saucepan and season with salt. Cook for 10 minutes (the cauliflower florets should still be slighty crunchy in the centre).
Make the béchamel sauce: place the butter in a saucepan and melt. Add the flour and mix with a spatula over a low heat for a few minutes. Start to add the milk a little at a time, mixing with a whisk. Cook for 5 minutes and then season with salt, pepper and nutmeg.
Butter a large baking dish and then add the cauliflower florets. Pour the béchamel sauce over the cauliflower and then sprinkle the grated gruyere over the top. Add a few small knobs of butter. Transfer to the oven and bake for 15 minutes.
«If you can't find gruyere cheese, use a good quality cheddar.»