My recipe:
Cut your cauliflower into florets and boil in salted water till just tender - around 6-8 minutes. Drain in a colander/sieve and let it steam dry
In a small saucepan, bring 300ml double cream up to the boil, then chuck in as much extra mature grated cheddar as you like (tip: at least half a pack!)
Simmer until the cheese has melted, and then season with black pepper and the juice of half a lemon
Add the cauliflower into an oven dish, pour the sauce over. Sprinkle some more grated cheddar on the top and then bake in a hot oven until the cheese on the top is golden brown.
This won't make a thick sauce, as if you had used flour, but it will be very tasty.
I sometimes add sliced leeks to this, that I've pan-fried in butter first. Or some sliced shallots and bacon lardons, also gently fried first. I sometimes add sliced tomatoes to the top.