Put all ingredients except water and oil in blender. If using onion, cut into chunks before adding. Blend on slow speed one minute then increase to medium high. Slowly pour in oil. When dressing begins to thicken gradually add water. Keeps in refrigerator for one day. Makes 2 cups or enough dressing for a salad to serve 8.
Chef's Showcase. (From The Corner House Restaurant, 501 Davenport Road, Toronto.)