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.