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.