This is a classic Italian sauce, originally from Genoa, that is used for hot pasta and pasta salads, or for flavoring. The southern French equivalent, pistou (made without pine nuts), is stirred into vegetable soups. To store pesto, put it in a glass jar and pour a thin layer of olive oil on top to seal it. Cover the jar tightly and refrigerate; it will keep for at least a week. Pesto also freezes particularly well, without loss of flavor or color.