Preheat the oven: 200°C fan/220°C/425°F/Gas Mark 7.
Cook the cauliflower: Do this in a large saucepan of boiling salted water for 5-7 minutes until the florets are almost, but not quite, cooked. (When you poke a knife into a stem it should be just tender.)
Meanwhile, make the cheese sauce: Measure the milk in a measuring jug. In a small bowl mix the cornflour with a splash of the milk from the jug. Use just enough to whisk the cornflour to a thick, lump-free paste (this feels a bit tricky, but it will come together eventually).
Pour the remaining milk into a small saucepan and place over a medium heat. Add the cornflour paste, whisk in, and cook for 5-10 minutes until the sauce starts to thicken, whisking often. Add the mustard, garlic granules, plenty of salt and pepper and the grated cheeses. Whisk and cook for another 5 minutes until the cheese is melted and the sauce is thick and smooth.
Drain the cauliflower, pour over the sauce: Make sure it's as dry as possible, and place in an ovenproof dish. Pour over the sauce and sprinkle the extra cheeses on top.
Time to bake: Bake in the preheated oven for 15-20 minutes until the cheese on top is melting and golden. You could also grill for a few minutes instead.
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