• Trim the stalk at the base so that the cauliflower can sit evenly upright in a casserole dish. Cut off any old outer leaves and leave the rest but trimmed so that they come halfway up the cauliflower.
• Make a deep cross in the base for even cooking. Brush the cauliflower top with oil, sprinkle over half a teaspoon of salt and half a teaspoon of pepper.
• Preheat the oven to 180 C / 350 F / 160 FAN / Gas 4.
• Place a lid on the casserole dish or cover tightly with foil. Bake for 1 hour and then remove the lid and cook for a further 20 minutes. Insert a skewer to make sure that the cauliflower is cooked all the way through.

For the cheese sauce

• Put the milk, flour and butter into a saucepan. Turn on a medium heat and start whisking straight away. Whisk continuously until the butter has melted. Keep whisking until it is smooth and thick then turn off the heat.
• Add 100g (3.5 ounces), of the cheese, freshly ground black pepper, mustard and a few grates of nutmeg. Stir the mixture until the cheese has melted. Check the seasoning and add more if necessary.
• Pour the cheese sauce over the cauliflower and sprinkle over the remaining cheese.
• Increase the oven heat to 200 C / 400 F / 180 FAN / Gas 6.
• Cook for another 20 minutes until golden and bubbling.
• Sprinkle over the chives and serve.