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To begin, preheat your oven to 200C/180C fan/gas 6. Boil the cauliflower in a large saucepan with salted water for 5 minutes. Add the broccoli and cook for an additional 3 minutes. Once done, drain the vegetables and transfer them to a large ovenproof dish.

Next, prepare the cheese sauce. Melt butter in a small pan until it starts to foam. Add flour and mustard powder, stirring constantly for 1 minute. Remove from heat and add milk, a little at a time, while stirring constantly to avoid lumps. Return the mixture to heat after adding half of the milk and add the remaining milk while continuing to stir. Bring it to a boil and then reduce the heat to a simmer and cook for 2 minutes until the sauce thickens and becomes smooth. Add most of the cheese and some seasoning, then take it off the heat and add the chives.

Pour the sauce over the vegetables and sprinkle the remaining cheese on top. You can prepare this dish a day ahead and store it in the fridge. Cook in the oven for 35-40 minutes until bubbling, adding an extra 5-10 minutes of cooking time if it's been in the fridge.