Scatter onion over the base of a 5.5L capacity slow cooker bowl. Arrange chicken on top. Pour over wine and stock. Add tarragon, garlic and dot over butter. Cover and cook on low for 6 hours.
Remove chicken from the cooking liquid and shred meat. Place in a serving dish and set aside. Strain cooking liquid and discard tarragon. Place onion over shredded meat. Transfer cooking liquid to a saucepan, reserving 2 tbsp. Place saucepan over medium heat and bring to the boil. Boil for 10-15 minutes or until reduced by two-thirds.
Combine the reserved cooking liquid and cornflour in a small bowl. Add cornflour mixture to stock mixture, stirring constantly, until mixture bubbles and thickens. Stir in cream, then bring to the boil. Reduce heat to simmer for 2-3 minutes or until sauce thickens slightly. Stir in parsley and season. Sprinkle over extra parsley.
Serve sauce over or alongside chicken with bread.
