In a bowl, cover the sun-dried tomatoes with boiling water. Let stand until softened, 20 minutes. Drain the tomatoes, reserving 2 tablespoons of the soaking liquid.
In a food processor, puree the yogurt, sun-dried tomatoes and the reserved soaking liquid with the oil, vinegar and ancho chile powder. Scrape the dressing into a bowl. Stir in the cornichons, onion and tarragon and season with salt and pepper.
Two Tablespoons 25 cal, 1 gm fat, 0.2 gm sat fat, 2 gm carb, 0 gm fiber.
Serve on romaine lettuce, as a crudité dip or spread on fish before broiling.