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Heat the oven to 220C/200C fan/gas 7. Tip the leeks into a steamer basket and steam for 6-8 mins until bright green and just becoming tender.
Meanwhile, heat the butter and flour together in a saucepan, stirring until you have a thick paste. Add the milk, a splash at a time, stirring continuously until all the milk has been added. Cook for 2-3 mins more until the sauce has thickened enough to coat the back of a spoon. Stir in most of the mature cheddar, the mustard and some seasoning.
Fold the steamed leeks into the cheese sauce, then tip everything into an ovenproof baking dish. Scatter over the breadcrumbs, thyme and remaining cheese, then bake for 20 mins until golden, crisp on top and bubbling at the edges. Leave to stand for 5 mins before serving.