Cauliflower boiled briefly, covered in a garlic béchamel and cheese, and finished under the grill. Half an hour, and draining the cauliflower properly is what keeps the sauce from going watery.
Drain the cauliflower properly and give it a minute to steam dry. Water clinging to the florets goes straight into the béchamel in the oven and turns a good sauce thin and grey. Boil it briefly too: cauliflower that is already soft before it goes into the dish collapses entirely during the baking. The béchamel is butter and flour in equal amounts cooked for a minute, then cold milk added while stirring, and nutmeg is the classic seasoning. The final three minutes under the grill is what makes it a gratin rather than a bake.