Process the first four ingredients in a food processor to a rough paste. With the machine running, slowly pour the olive oil through the tube. If the sauce seems dry (it should be a thick paste) add a little more olive oil. Season with salt and pepper to taste. Use immediately or store in a covered jar (press plastic wrap directly onto the pesto to prevent discoloration) in the refrigerator for up to one week. Pesto may be frozen up to 2 months.