Place the chicken breasts into the bottom of a large slow cooker. Sprinkle the salt and pepper over the chicken. Add the onion, carrots, celery, and garlic on top. Pour in the chicken stock.
Cover and cook on LOW for 4 hours or HIGH for 3 hours.
Take the chicken breasts out of the slow cooker and shred them. Add the chicken back to the crock, give everything a taste, and add salt and pepper as needed.
Break the dumplings into thirds (yes, the dumplings should still be frozen when you break them). Add the dumpling pieces one at a time to the pot, stirring them into the soup every 5 pieces or so.
Place the slow cooker on HIGH and cook for another 1 hour to 1 hour 30 minutes, stirring every 15 minutes. This will help prevent the dumplings from sticking together or sticking to the pot.
Dinner is ready when the dumplings are tender and puffed up, and the broth has thickened. Portion the chicken and dumplings into bowls and garnish with chopped parsley if desired before serving warm.
Notes
This dish will get very thick if you leave it sitting. Add more chicken stock to loosen it up if needed.