Lightly spray a 6-quart slow cooker with nonstick cooking spray.
Place the chicken into the bottom of the slow cooker.
Season the chicken with the salt, black pepper, and garlic powder.
Sprinkle the diced onion over the chicken and add the butter.
Pour in the chicken broth.
Add the cream of chicken soup, chicken gravy mix and poultry seasoning. Stir gently around the chicken until combined.
Add the cubed cream cheese.
Cover and cook on LOW for 3-4 hours or HIGH for 2½–3 hours, until the chicken is cooked through and tender.
Stir in the thawed shredded hash browns, frozen peas and carrots and frozen corn until well combined.
Cover and cook on LOW for 1½–2 hours, stirring once halfway through, until the hash browns are tender and the cream cheese has melted.
Stir in 2 cups of the shredded cheddar cheese until completely melted and creamy.
In a small bowl, combine the crushed Ritz crackers with the melted butter.
Sprinkle the remaining ½ cup cheddar cheese evenly over the casserole.
Top with the buttery Ritz cracker crumbs.
Cover and cook for another 10–15 minutes until the cheese is melted and everything is hot.
Sprinkle with chopped parsley before serving.