You will need a food processor. If you’re using the pesto right away to dress maccheroni alla chitarra (or other pasta), heat a large pot of salted water to a boil before processing the sauce. To make the pesto: Heap the basil, parsley, garlic, walnuts, and salt into the food processor. Pulse several times, to chop everything together coarsely, then, with the machine running, pour in the ½ cup olive oil in a slow, steady stream. Stop and scrape down the sides of the work bowl, and process to a uniformly fine bright-green pesto. Put all the pesto into a large, deep bowl, big enough to toss all the pasta in. Meanwhile, start cooking the maccheroni or other pasta. Shortly before it is done, ladle ½ cup or so of the boiling pasta water into the bowl, and stir to warm and loosen the pesto—use only as much water as needed to bring the pesto to tossing consistency. When the maccheroni is perfectly al dente, lift it from the pot with tongs and a spider, drain for an instant, then drop it into the bowl. Toss pasta and sauce together thoroughly. Sprinkle a cup or so of grated cheese over it, then toss again. Serve immediately in warm bowls, with more cheese at the table. To store and use the pesto later: Scrape it from the food processor into a small jar or container. Smooth the top surface, and cover it with a thin layer of olive oil or a piece of plastic wrap to prevent discoloration. Refrigerate for a week, or freeze for several months; warm to room temperature before using.