Pat pork dry. Score well with sharp knife. Combine parsley, onion, peel, basil, and garlic in a small bowl. Whisk in 2/3 of oil. Rub into pork. Wrap in foil and refrigerate overnight.

Let pork stand at room temperature 1 hour before roasting. Preheat oven to 350 degrees F. Brush pork with remaining olive oil. Set on rack in shallow pan. Roast until meat thermometer inserted in thickest part of meat registers 170 degrees F, about 2 1/2 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.