Preparation Cook spaghetti in a 6-quart pot of boiling salted water until al dente. While pasta is boiling, blend parsley, sage, and garlic with salt in a food processor until finely chopped. Add walnuts and pulse until finely chopped. With motor running, add oil in a steady stream. Turn off motor, then add 1/3 cup cheese and pepper and pulse to combine. Reserve 1 cup cooking water, then drain pasta in a colander. Thin pesto with reserved cooking water in a serving bowl, then add pasta and toss to combine. Sprinkle with cheese before serving.
Preparation Cook spaghetti in a 6-quart pot of boiling salted water until al dente. While pasta is boiling, blend parsley, sage, and garlic with salt in a food processor until finely chopped. Add walnuts and pulse until finely chopped. With motor running, add oil in a steady stream. Turn off motor, then add 1/3 cup cheese and pepper and pulse to combine. Reserve 1 cup cooking water, then drain pasta in a colander. Thin pesto with reserved cooking water in a serving bowl, then add pasta and toss to combine. Sprinkle with cheese before serving.