Heat the water (approx. 3 l) for the pasta in a saucepan, add salt when it begins to boil, then add the pasta and cook according to the package instructions, but try 3 minutes before the end of the cooking time.
It really must be al dente, because it will continue to cook while the sauce is being prepared.
Before draining the pasta, remove 1 cup of cooking water, then put the drained pasta with 2 tablespoons of the liquid, the crumbled cheese and the spinach back into the pot and season with some coarsely ground pepper.
Put the lid on, remove the pot from the heat and let the pasta rest for 2 minutes.
Put the pot without lid back on the stove and mix pasta, cheese and spinach over low heat, adding as much of the collected cooking liquid as you need - I find 100 ml altogether about adequate - until the cheese has melted into a light sauce and the spinach has collapsed.
Remove the pasta from the heat, mix with 2 tablespoons of the chopped pistachios and divide between two preheated plates. Sprinkle each portion with half of the remaining pistachios and serve immediately.