Melt the butter in a large saucepan, stir in the plain flour and cook for a few minutes. Lower the heat and gradually whisk in the milk, stirring constantly and making sure there are no lumps.
Keep stirring until the sauce has thickened, this can take several minutes. Add the mustard, then add the cheddar cheese, then keep stirring until the cheese has melted. Season with pepper.
Meanwhile, boil the macaroni in a large pan of salted boiling water for about 10 minutes until they are al dente.
In another pan, also boil the cauliflower florets until they're soft. Once cooked, add them to the cheesy sauce and using a stick blender blitz until smooth. If you don't have a stick blender you can use a standing blender or food processor.
Preheat the oven to 200°C/Fan 180°C. Combine the cooked macaroni into the cheesy cauliflower sauce and pour everything into a large ovenproof dish. Sprinkle the panko breadcrumbs on top evenly, as well as the parmesan and cheddar cheese
Put in the oven and bake for around 25-30 minutes until bubbling. Serve with a green salad and some garlic bread.