Preheat oven to 170°C/340°F/Gas Mark 3–4. Lightly toast pistachios, then cool. Place all but 12 pistachios in a food processor with mint, parsley and garlic. Process 5 seconds. Add half the oil and process another 5 seconds. Add about two-thirds of the parmesan and remaining oil. Process 5 seconds. Check seasoning and add salt and pepper if desired. The flavour will get diluted once you add the pasta so make it quite strong. Tip into a large bowl.
Bring a large saucepan of salted water to the boil and cook pasta until al dente (8–10 minutes). Scoop out 3–4 tablespoons of cooking water and mix into the pesto in the bowl. Tip pasta into a colander to drain, and then into the bowl and toss with the pesto. Serve in bowls with remaining parmesan and pistachios sprinkled on top.