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Melt the butter in a saucepan, then stir in the flour and mustard powder and cook for 1 min. Gradually stir in the milk until smooth with no lumps, then keep stirring until the mixture begins to bubble and thickens to a creamy sauce. Remove from the heat, then stir in all but a handful of the grated cheese.
Heat oven to 220C/200C fan/gas 7 and bring a large pan of water to the boil. Cook the potato slices for 5 mins, tip in the broccoli and cauliflower for another 3 mins, then finally add the peas for 1 more min. Drain all the veg and pat dry. Reserve enough potato slices to cover the top of the finished dish, then gently stir the rest of the vegetables into the sauce with the chives.
Tip into a deep ovenproof dish, arrange over the reserved potato slices, then sprinkle with remaining cheddar. Bake for 20-25 mins until the topping is golden and crisp, then serve straight from the dish.
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