Bring a large pan of water to the boil. Prepare the cauliflower by cutting it into bite‑sized florets
Boil the cauliflower for 10–12 minutes, or until tender. Drain the cauliflower, keeping the cooking water - you should have around 900ml left.
Grate 130g cheddar cheese and finely dice the chives
In a large pan, melt the butter with a small squirt of cooking oil (this stops the butter burning). Add the flour to make a roux and cook on a low heat for 3 minutes, stirring
Add the milk little by little, whisking vigorously to avoid lumps. It will become very thick, like a paste - keep whisking until smooth before adding more.
Start adding the reserved cauliflower water gradually, whisking continuously. Keep the pan on the heat and whisk vigorously to maintain a smooth sauce. Add enough water to reach your desired thickness - around 700ml works well.
Add most of the grated cheese, holding back a handful for the topping. Whisk until fully melted and smooth. Season the sauce with salt and pepper, then stir in the chives.
Add the cooked cauliflower to the sauce and gently fold it through
Transfer everything to a baking dish, sprinkle over the remaining cheese, and bake at 200°C for 20 minutes, or until golden, bubbling and crispy on top.