Tip the nuts into a large non-stick pan, and cook over a low heat to toast for a few minutes, then remove to a plate. Return the pan to the heat and add the oil. Stir in the leeks and cook for 10 mins, stirring frequently, until softened. Meanwhile, boil the penne for 12 mins or until al dente.

Stir the garlic into the leeks then add the spinach and cook until wilted. Drain the pasta, but keep back a mugful of the water. Tip roughly half the water into the vegetables along with the pasta and half the cheese, then stir well over the heat to emulsify everything. Toss through the last of the cheese with the toasted walnuts and serve.