Heat oven to 400° F. Trim the top 1/4 of each artichoke. Remove and discard the tough, dark green outer leaves until you can see the pale green heart underneath. Trim the entire stem. Cut each artichoke in half lengthwise. In a large cast-iron skillet, combine the parsley, garlic, oil, salt, pepper, and 1/4 cup water. Add the artichokes and toss. Roast, stirring twice, until the artichokes are tender, about 30 minutes. Sprinkle with the zest. Serve with the mayonnaise (if desired).