In a food processor, chop the pistachios. Add the olive oil, mint and garlic and pulse to combine. Transfer to a bowl, stir in the 1/2 cup of cheese and the scallions; season with salt.

In a large pot of salted boiling water, cook the pasta until al dente; drain, reserving 1/2 cup of the cooking water. Return the pasta to the pot. Add the cooking water and the pesto and cook over low heat, tossing, until coated. Serve, passing more cheese at the table.