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.
The secret to making pesto into a great pasta dressing is to get the sauce to the right consistency. Fresh from the food processor, pestos are usually too thick to coat pastas; you’ll just end up with mounds of the sauce amid a lot of undressed noodles. You need to mix in just enough of the pasta cooking liquid to get the pesto to coat the pasta, but not so much that you’ve created pesto soup.
