Add the chicken, onion, potatoes, and mixed vegetables to your slow cooker.
Whisk together the cream of chicken soup, cream of celery soup, milk, garlic powder, poultry seasoning, onion powder, and pepper until well combined.
Pour the mixture evenly over the chicken and vegetables.
Cover and cook on low for 6–8 hours or on high for 4 hours.
About 30 minutes before serving, remove the chicken, shred it, and return it to the slow cooker. Stir to combine.
Cut the biscuits into small pieces and sprinkle them evenly over the top of the chicken mixture.
Melt the butter and brush it over the biscuit pieces.
Cover and cook for the remaining 30 minutes, or until the biscuits are cooked through.
Garnish with parsley, if desired.