• Preheat the oven to 200℃/400F fan, 220℃/450F or Gas Mark 7.
• Cook the cauliflower 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.)
• While the cauliflower is cooking, start making 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 well (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.
• Bake in the preheated oven for 15-20 minutes until the cheese on top is melting and golden.

To make ahead:

• Prepare to the end of step 4. Cover and store in the fridge for up to 48 hours until you're ready to cook. Finish in the oven as per step 5, allowing an extra 5-10 minutes' cooking time (because it will be very cold from the fridge). Ensure the centre is piping hot before serving.