Place onions on bottom of a 6-qt. slow cooker. In a small bowl, mix butter and lemon peel.
Tuck wings under chicken; tie drumsticks together. With fingers, carefully loosen skin from chicken breast; rub butter mixture under the skin. Secure skin to underside of breast with toothpicks. Place chicken over onions, breast side up. Add stock, parsley and dill.
Drizzle lemon juice over chicken; sprinkle with seasonings. Cook, covered, on low 4-5 hours (a thermometer inserted in thigh should read at least 170°).
Remove chicken from slow cooker; tent with foil. Let stand 15 minutes before carving. Yield: 6 servings.