Place the chicken breasts in slow cooker. Season with salt and pepper.
Add chicken broth.
Add potatoes, frozen veggies, onion, and celery stalk. Stir to mix ingredients.
Place lid on the slow cooker and cook on low for 6-8 hours.
Thirty minutes before serving, combine cold milk and cornstarch. Whisk to remove lumps. Set aside.
Next, remove chicken from slow cooker and shred with a fork.
Pour milk mixture and shredded chicken into slow cooker and stir to combine.
Turn slow cooker to high and heat until simmering, stirring occasionally. Allow the soup to simmer for 5 minutes, then turn down heat and let soup rest for 5 minutes before serving.