Pesto is a famous sauce from Italy, traditionally made by crushing fresh basil with garlic and oil, using a mortar and pestle—the word “pesto” means “to pound”. A blender makes it easy! This sauce is delicious tossed with hot or cold pasta or potatoes, as a spread for sandwiches, or served as a sauce with vegetables, fish or chicken.
Toast the walnuts if using – place in a skillet over medium heat and cook, stirring occasionally, until light golden brown. Remove from heat and transfer to plate to cool.
Wash the arugula by placing in a large bowl of cold water; swish gently to remove any dirt. Lift the arugula out of the water. Dry on paper towels, in a strainer, or in a salad spinner.
Put garlic and walnuts into a blender. Blend 3-4 seconds to finely chop the garlic and nuts.
Add the arugula, basil or parsley leaves if using, lemon juice and oil to the blender. Blend, scraping down the sides with a spatula several times, until all the ingredients are processed into a thick sauce.
Add the Parmesan cheese, and blend 2-3 seconds, just to combine all the ingredients.
Refrigerate for at least one hour to allow flavors to develop. Store tightly covered in the refrigerator for up to one week.