Heat water in a pan for the pasta, salting it when it comes to the boil, then add the pasta and cook according to packet instructions, but checking 3 minutes before it’s meant to be done. This needs to be really al dente because it will carry on cooking as you make the sauce.
Before draining the pasta, remove a cupful of pasta-cooking liquid, then tip the drained pasta back in the hot pan with 2 tablespoonfuls of the liquid, the crumbled cheese and the baby spinach, and give a good grinding of coarse black pepper. Put the lid on the pan — off the heat, though back on the stove — and leave to stand for 2 minutes.
Remove the lid, turn the heat back on low, and stir the pasta, cheese and spinach together, along with as much of the cupful of cooking liquid as you need — I find 100ml / a scant ½ cup total is about right — until the cheese is melted into a light sauce and the spinach wilted.
Take off the heat, toss with about two-thirds of the chopped pistachios and divide between 2 warmed bowls, sprinkling each bowl with the remaining nuts. Serve immediately.
