Place the chicken thighs in the slow cooker. Pour in the chicken broth, then add the sliced carrots, chopped celery, and chopped onion.
Sprinkle the garlic powder, dried thyme, salt, and black pepper over the top. Add the cream of chicken soup and 1 cup of milk, stirring gently to combine all the ingredients.
Cover the slow cooker and cook on low for 6 hours, or until the chicken is fully cooked and tender.
After 6 hours, remove the chicken from the slow cooker and shred it using two forks. Return the shredded chicken back to the slow cooker. Stir in the frozen peas.
In a large mixing bowl, combine the biscuit mix and ⅔ cup of milk to make the dumpling batter. Stir until just combined.
Drop spoonfuls of the dumpling batter over the hot chicken mixture in the slow cooker. Cover the slow cooker and cook on high for an additional 30 minutes, or until the dumplings are cooked through and fluffy.
Once the dumplings are done, garnish with fresh parsley if desired and serve warm.