Cook the macaroni following pack instructions, drain and keep warm under a lid. Or, this can be done well ahead, then rest in the fridge.
Tear the cauliflower into florets, then cut the stem into small pieces. Steam theses over boiling water for about 5 mins or blanch them in boiling water for 3 minutes. Darin and keep warm.
Melt the butter in a pan, then add the cornflour and stir for 2 mins. Add the bay leaf and gradually add the milk, stirring all the time to get a smooth sauce.
Add the grated Cheddar and mustard, season with salt and peppers.
Stir until you get a thick sauce, then add the cauliflower and the macaroni. Combine well.
Preheat the oven to 180°C.
Transfer the mixture to a lightly buttered ovenproof dish and remove the bay leaf.
Scatter some Parmesan or extra Cheddar on top.
Put the dish in the oven and bake for 20 minutes until golden and bubbling.