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If you are eating one of the pasta bakes now, heat oven to 190C/170C fan/ gas 5. Boil the pasta with the leeks for 10 mins, then drain.
Meanwhile, put the butter, flour, mustard, nutmeg and milk in a large pan with some seasoning. Gently heat, stirring all the time, until bubbling and thickened, then cook for 2 mins more, stirring frequently. Remove from the heat and stir in two-thirds of the cheese. Toss the remaining cheese with the bread and walnuts.
Cook the spinach in the microwave, or pour a kettle of boiling water over it to wilt, then squeeze out the excess water. Stir into the sauce with the pasta, leeks and some seasoning. If necessary, add a little extra milk to loosen. Divide between 2 ovenproof dishes and scatter the bread mixture on top. If eating straight away, bake for 40 mins until golden, or cool to freeze.