Cooks' note:
To serve with pasta, put pesto in a large bowl; meanwhile, cook 1 pound spaghetti until al dente. Whisk about 1/2 cup pasta-cooking water into pesto. Drain pasta, then toss with pesto and salt and pepper to taste.
To serve with pasta, put pesto in a large bowl; meanwhile, cook 1 pound spaghetti until al dente. Whisk about 1/2 cup pasta-cooking water into pesto. Drain pasta, then toss with pesto and salt and pepper to taste.