Season duck liberally with salt and pepper. Combine next 4 ingredients in a plastic bag. Add duck breasts and shake bag to coat evenly. Heat oil in a skillet over medium-high heat. Sear duck breasts on both sides until medium-rare. Remove from pan and let stand for 5 minutes. To prepare dressing, add first 5 ingredients to a blender or food processor and process until smooth. While motor is running, add oil slowly in a thin stream until dressing is smooth and oil is emulsified. Add avocado and tarragon and process until smooth (have I used the word “smooth” enough yet?). Season with salt and pepper.