Melt the butter
with the oil in a large pan, tip in the
leeks and splash in a little hot water.
Cover and cook over a low heat,
stirring occasionally, for about 10
minutes until no longer squeaky.
While the leeks
are cooking, boil the penne in salted
water according to the packet
instructions. Meanwhile, tip the
crème fraîche and mustard into the
leeks, add three-quarters of the
cheese and season well. Stir until
the cheese melts. Take off the heat.
pasta, reserving the cooking water.
Tip the pasta into the sauce and
stir, adding enough cooking water
to make the sauce coat the pasta.
Stir in the spinach, a big handful at
a time, until it wilts, splashing in a
little more water if needed. Toss in
the remaining cheese and serve.