Bring a large saucepan of water to the boil, then add 1 large cauliflower, broken into pieces, and cook for 5 mins – lift out a piece to test, it should be cooked.
Drain the cauliflower, then tip into an ovenproof dish. Heat oven to 220C/200C/gas 7.
Put the saucepan back on the heat and add 500ml milk, 4 tbsp flour and 50g butter.
Keep whisking fast as the butter melts and the mixture comes to the boil – the flour will disappear and the sauce will begin to thicken. Whisk for 2 mins while the sauce bubbles and becomes nice and thick.
Turn off the heat, stir in most of the 100g grated cheddar cheese and pour over the cauliflower. Scatter over the remaining cheese, and 2-3 tbsp breadcrumbs if using.
Put in the oven and bake for 20 mins until bubbling.
