In The Bowl of a food processor, blend the arugula and garlic until finely chopped. With the machine running, gradually add the oil, processing until well blended. Transfer the pesto to a large bowl, and stir in the Parmesan cheese, ½ teaspoon of salt, and ¼ teaspoon of pepper. Season the pesto with more salt and pepper to taste. (The pesto can be made 2 days ahead. Cover and refrigerate.)