Heat the oven to 210C/190C fan/gas 7. Tip the cauliflower into a shallow baking dish and toss with the oil, cayenne and
a pinch of salt. Scatter with half the parmesan and roast for 20 mins until crisp and golden. Toss the florets, then sprinkle with more (but not all) of the parmesan and roast for 10 mins more until starting to char a little. Set aside.
While the cauliflower is roasting, make the sauce. Heat the butter in a saucepan over a medium heat until frothy. Scatter over and stir in the flour, cooking for 3 mins until you have a sandy paste, then add the milk, a splash at a time, keeping it at a simmer and whisking between each addition until smooth. Once all the milk has been added, continue to cook the sauce gently, stirring occasionally for 15 mins until thickened and silky, then stir through the mustard powder and yeast extract, if using. Remove from the heat and add the grated cheddar, a handful at a time, until you have a thick, cheesy sauce. At this point stir in the cubed cheese and season with nutmeg and salt, if needed.
Pour the sauce over the cauliflower in the baking dish to fully coat it. Mix the breadcrumbs with the remaining parmesan, then scatter this over the cauliflower cheese and bake for 20 mins. Sprinkle with the crispy onions and bake for a further 10 mins until bubbling and deep golden. Serve in warm and melting scoops from the baking dish.
