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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, sundried tomatoes and mustard into the leeks, add three-quarters of the cheese and season well. Stir until the cheese melts. Take off the heat.
Drain the 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.