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Preheat the oven to 200oC
Cook the kale in a large pan of boiling water for 3 minutes, remove with a slotted spoon and cool under cold water, squeeze out the excess liquid. Add the macaroni to the boiling water and cook for 9-10 minutes, adding the leek for the last 2 minutes. Drain well and return to the pan.
Meanwhile, melt the butter in a small pan, stir in the flour and cook for 1 minute. Off the heat gradually blend in the milk, then bring to the boil, stirring until thickened. Stir in 100g cheese and the mustard, season well.
Stir the sauce into the macaroni with the kale and transfer to an ovenproof serving dish, sprinkle with remaining cheese and bake for 25 minutes until golden.