add the chicken, carrots, celery, onion, garlic, rice, seasonings, and chicken broth to the crock of your slow cooker, stir to combine
cook on low without stirring for 6-8 hours
once the chicken is cooked, remove it from the pot and shred
while the chicken is out of the pot add the milk
mix the water and cornstarch together
add a ladle of hot soup to the cold cornstarch and stir it together to temper it(keep it from seizing up)
then pour it into the hot soup and stir to combine
return the chicken to the pot and allow the soup to thicken (about 30 mins)
when thickened up serve hot