Good-quality cheese is essential for rich, full-flavored pesto. Seek out true Italian Parmesan, as well as pecorino Sardo, a sheep's milk cheese from Sardinia. If you can't find pecorino Sardo, don't use pecorino romano, which is too strong; instead, opt for Manchego, a Spanish sheep's milk cheese. Roughly chopping the basil by hand before adding it to the food processor minimizes the mechanical action needed to break down the leaves so the pesto won't become too smooth. To store pesto, press a piece of plastic wrap against its surface and refrigerate up to 3 days.