Pour the broth into a 6- to 7-qt. slow cooker. In a small bowl, combine the mustard, 1 tsp. salt and 1/8 tsp. pepper; rub the chicken with the mustard. Place the chicken in the slow cooker and top with the garlic and 2 tbsp. tarragon. Cover and cook on low heat until the chicken is tender and an instant-read thermometer inserted into a thigh away from the bone registers 165 degrees , about 5 hours.
Transfer the chicken to a serving plate and cover to keep warm. Strain the pan juices into a small saucepan and bring to a simmer over high heat. In a small bowl, combine the cream and cornstarch. Whisk the mixture into the saucepan and cook until the sauce is thickened; stir in the lemon juice. Pour the sauce over the chicken.
In a small bowl, combine the lemon zest, parsley, and 1 tbsp. tarragon. Top the chicken with the zest-herb mixture.