1. 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.
2. 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.