Cut the cauliflower, and cook it for 15 minutes in salted boiling water.
Arrange it into a greased dish.
In a pot, melt the butter, and add the flour all in once. Whisk.
Gradually pour the milk, stirring well with a whisk. The mix should thicken gradually. Once it got the right texture, take out of the heat.
Pour on the cauliflower. Sprinkle with grated cheese.
Bake 30 minutes at 350°F (180°C).
There you are, your cauliflower gratin is ready !