Step 1 To make the pistou in a food processor, turn on a processor and drop in the garlic. Scrape down the sides of the work bowl, add the basil and salt to taste (1/4 to 1/2 teaspoon) and process until finely chopped. Scrape down the sides once more and turn on the machine. Drizzle in the oil with the machine running, then drop in the tomato if using. Process to a paste. Taste and add more salt, if necessary. Stir in the Parmesan. To make the pistou in a mortar and pestle, pound the garlic with the salt to a paste. Add the basil, a handful at a time, and pound and grind the basil to a paste. Add the optional tomato and work to a paste with the basil. Drop by drop, work in the olive oil. Stir in the cheese.