Place all ingredients in blender and blend until very smooth. (A heavy-duty blender works best for this recipe; other blenders may not get the mixture smooth enough.)
Stir in finely diced onion.
Notes
For best results, remove dressing from refrigerator 20 minutes before serving. When dressing is chilled it becomes thick; removing it from refrigerator allows it to return to normal consistency.Yield: 1 1/3 cups