Adapted from a recipe from The Gourmet Cookbook. Original recipe used buttermilk, I have substituted plain kefir and made a couple of other minor changes. The final product is a healthier, lighter, "buttermilk" style dressing.
Put all of the ingredients, except the olive oil in a blender or food processor. Puree until all ingredients are well incorporated. Drizzle in the olive oil while processing. Continue to blend for another 20 seconds or so.
Best if refrigerated overnight, as it allows the dressing to thicken slightly, and the flavors to mingle. I have, however, used it right out of the blender and it still tastes great.