Add arugula, walnuts, garlic, parsley, red pepper flakes to food processor.
Using a microplane, grated peel of both lemons into food processor.
Process until forms a paste, slowly add olive oil and the juice and pulp of the lemons while machine is running.
Process until well blended. With machine stopped, add salt and pepper to taste. If mixture is too thick, you can add more olive oil or warm water. Add grated parmesan.
Notes
Keep in air tight container in the refrigerator. To keep the pesto fresh, top off with olive oil before storing.
