Finally, stir in three tablespoons of oil. The finished sauce, a thick, rich paste, is best stored at room temperature. Made with cheese, the sauce will last two or three days at room temperature and four to seven days in the refrigerator. Fresh basil is available year-round, after all, and you can make pesto whenever you want. It would be a romantic fiction to claim that most Ligurian cooks make pesto by hand.
People use the blender.
Not the processor, which never gets the consistency right and is disastrous with the nuts, although it is acceptable for shredding basil leaves. In the coastal town of Recco, which Plotkin considers to be arguably the epicenter of Ligurian cuisine, the cooks at Manuelina restaurant laughed when I asked to see the mortar in which they had prepared the pesto for our lunch. In one of those enviable Italian gestures they rolled their hands to indicate eternal and ridiculous effort and looked toward the blender. Stir in the cheese by hand, tasting the sauce as you add it. Every Ligurian cook I observed diluted pesto with starchy cooking water, usually in a ratio of three parts pesto to one part water.