Put a large pan of salted water on to boil, and cook the macaroni together with the cauliflower for 10 minutes (make sure you don't overcook - you want them to be a bit al dente when you put the mac 'n' cheese in the oven).
Pre-heat your oven to about 160C.
Melt the butter in a pan and stir in the flour using a whisk. Stir and heat for about a minute, then turn the heat off and gradually add the milk, whisking all the time. If you use a whisk, you'll find it doesn't get lumpy.
When all of the milk has been added, turn the heat back on low and heat through, whisking all of the time, until the sauce thickens to your liking then turn the heat back off.
Stir the chopped, raw onion, bay leaves, mustard and grated cheddar cheese into the sauce and season with salt and plenty of black pepper.
Transfer the cauliflower, macaroni and sauce to an oven proof dish and combine. Top with additional grated cheese (I find that gruyere is very good as a topping) and bread crumbs (rub the stale bread between your palms to create the bread crumbs).
Pop in the oven for about 40 mins, or until the topping is brown, crispy and bubbling and you can't resist tucking in!