1. Place potatoes in a 4- to 6-quart slow cooker. Add chicken, leeks, wine, and salt.
2. Cover; cook on high for 3–4 hours or on low for 6–7 hours until chicken and potatoes are tender.
3. Transfer chicken and all but 4 potatoes evenly to plates. Use a fork to smash remaining potatoes into cooking liquid.
4. Add peas, milk, and yogurt; cook for about 10 minutes or just until heated through. Spoon over chicken, and sprinkle with tarragon.