Bring a large pan of salted water to the boil. Pull any large leaves off the cauliflower, but leave any smaller ones attached. Carefully lower the cauliflower into the water and boil for 15 mins, then drain and set aside.

Heat the oven to 180C/160C fan/gas 4. Melt the butter in a medium pan over a medium heat, then stir in the flour and paprika, and cook for 1-2 mins. Remove from the heat and slowly whisk in the milk until smooth, then mix in the mustard. Return the pan to the heat and bring the mixture to the boil, whisking continuously. Add two-thirds of the cheese and whisk until the cheese has melted. Season to taste.

Put the cauliflower in the middle of a deep baking dish and pour over the cheese sauce. Sprinkle with the remaining cheese. Bake uncovered for 30-35 mins until golden brown and bubbling. Scatter with the chives before serving.