In a food processor or blender, process the egg and mustard until blended. With the motor running, slowly pour the oil through the feed tube in a steady stream. The mixture will thicken. Add the honey, cayenne, salt, parsley, and vinegar and pulse to blend.
Transfer to an airtight container and refrigerate for at least 30 minutes before using. This is best used within 24 hours.
Yield: 1 1/2 cups