The most basic, most traditional pesto is this basil-based one that was invented in Genoa, in northern Italy; it’s bright green and wonderfully aromatic. Be sure to add your oil slowly, so that the sauce fully emulsifies—that is, all the ingredients bind together, creating a thick, uniform consistency. Lots of basil varieties are available, some sweeter, some spicier, and in fact basil is an important ingredient in the cooking of southeast Asia, especially in curries. Each variety will impart its unique flavor to a pesto, so be adventurous, and try whatever type appeals to you. Just remember: You need a lot of it, and it should be fresh and crisp, not wilted or soggy.