Cook the spaghetti as usual until al dente. Roast the chopped pistachios in a little oil, then add the water and simmer a little, then stir in the crème fraîche and Parmesan. If the sauce is too thick, thin it with a little more water.
Bring to the boil briefly and season to taste with salt, pepper and nutmeg.
Drain the spaghetti, allow to drain briefly, mix with the sauce and serve immediately.