Toss chicken, carrots, celery, onion, garlic, tomatoes, potatoes, chicken stock, tomato paste, wine, fennel seeds, salt and rosemary into the slow cooker.
Cook on the lowest setting for 4-6 hours.
During the last 30 minutes of cook time, combine cornstarch with water and mix until no lumps remain, add to the slow cooker along with balsamic vinegar.
Turn heat to high, and cook for 30 additional minutes, until slightly thickened.
Transfer chicken thighs to a clean bowl and shred with two forks or an electric hand mixer. Return to the stew and mix in.