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Bring a pan of lightly salted water to the boil and cook the cauliflower florets for 3 mins. Add the broccoli and cook for 2 mins more. Drain in a colander and leave to steam-dry.
Tip the spinach into a large saucepan, add 2 tbsp water and cook with the lid on over a low heat for 3-4 mins until wilted, then tip into a sieve and press the excess moisture out of it with the back of a wooden spoon. Set aside.
Heat the oven to 220C/200C fan/gas 7. Melt the butter in a saucepan over a low heat until foaming, then stir in the flour to create a thick paste. Cook for 2 mins, remove the pan from the heat and whisk in the milk in several additions until lump-free. Return to the heat and whisk until thickened. Whisk in the mustard, most of the cheese and the chives. Season to taste.
Arrange the vegetables, including the cauliflower leaves, in a medium ovenproof dish. Pour over the sauce, then sprinkle with the remaining cheese. Bake for 25-30 mins or until golden and bubbling.