Heat the oven to 200C/fan 180C/gas 6 and bring a large pan of salted water to the boil. Add the cauliflower heads to the pan and boil for 10 minutes. Drain well, then leave to steam dry for a few minutes, before brushing the heads with 1 tbsp oil and roasting for 30 minutes. Arrange in a baking dish.

Put the onions, bay and butter in a saucepan with a little salt, and fry gently until soft and golden. Stir in the flour with the mustard powder until no dusty bits remain, then very gradually stir in the milk to make a smooth sauce. Bubble gently for 4-5 minutes to thicken, then stir in the cream until heated through. Take off the heat, add the gruyère, emmental and 2/3 of the parmesan and stir until they have melted, then season with some nutmeg, white pepper and salt. Put back on a low heat if you need to, for the cheese to melt. Pour over the cauliflower heads and chill for up to two days, if you like.

Heat the oven to 220C/fan 200C/gas 7, sprinkle the gratin with the remaining parmesan and a little more nutmeg, and bake for 20 minutes until bubbling and golden on top.