Layer the diced onion in the bottom of the crockpot.
Place the chicken thighs on top of the onion in a single layer.
In a medium bowl, combine the cream of chicken soup, cream of celery soup, poultry seasoning, chopped parsley, and black pepper. Mix well.
Pour the soup mixture evenly over the chicken in the crockpot.
Pour 2 cups of chicken broth over the chicken and soup mixture.
Cover and cook on high for 5 hours. Avoid opening the lid until 1 hour remains on the cooking time.
With 1 hour left, add the defrosted mixed vegetables to the crockpot. Stir to combine and shred the chicken slightly as you mix.
Open the can of refrigerated biscuits and flatten each one. Cut each biscuit into 4 long slices to create the dumplings.
Layer the biscuit slices over the chicken and vegetable mixture in the crockpot.
Cover and cook for the remaining hour. After cooking, stir gently to incorporate the dumplings into the chicken mixture.
Serve hot, garnished with additional chopped parsley if desired.
Enjoy!