Pat pork dry. Score well with sharp knife. Combine parsley, onion, peel, basil, and garlic in a small bowl. Whisk in two third of oil. Rub into pork. Wrap in foil and refrigerate overnight. Let pork stand at room temperature 1 hour before roasting. Preheat oven to 180ºC . Brush pork with remaining olive oil. Set on rack in shallow pan. Roast until meat thermometer inserted in thickest part of meat registers 80ºC , about 2.5 hours. Set meat aside. Degrease pan juices. Blend Sherry into pan juices. Cover and cook over low heat 2 minutes. Pour into sauceboat. Transfer pork to platter. Garnish with fresh parsley and lemon slices. Serve sauce separately.