Wash chicken and pat dry with paper towels. In a small bowl, combine salt, pepper, paprika, garlic powder, onion powder, thyme, cayenne powder, and brown sugar. Set aside ½ tablespoon of spice mixture. Completely coat chicken with the remaining spice rub. Loosen the skin from the breast meat and rub the spices under the skin as well.
Quarter half of one of the onions and roll in the reserved spice mixture. Stuff the cavity of the chicken with the onion and leafy celery stocks and carrots.
Spray the crockpot with nonstick cooking spray. Chop the remaining onion and celery and spread in the bottom of the crockpot. Place the chicken on top of the vegetables, breast side up. Cook on high for 3 ½ - 4 hours or until meat thermometer placed in the thickest part of the thigh reads 165℉ and juices run clear.
When chicken is finished cooking, remove chicken from crockpot and place on a foil lined pan. Tent with foil and let rest.
Using a fat separator, drain fat from the chicken drippings and discard. Return celery and onions to the crockpot. Add stuffing mix and chopped parsley and stir. Gently mix in 1 ⅓ cup of chicken drippings. Place lid back on crockpot and leave on high for 8 minutes.
Set oven to broil. Broil chicken on each side for 2-3 minutes to crisp up the skin, watching carefully to prevent burning.
To make gravy, place remaining chicken broth drippings (you should have about 1 ½ cups) in a small pan and bring to a boil. In a small bowl, stir together cornstarch and water until smooth. Slowly whisk cornstarch mixture into chicken drippings and let boil until thickened.