Lightly grease the inside of a 6-quart slow cooker. Arrange the chicken breasts in a single layer at the bottom. Season with garlic powder, black pepper, thyme, and salt.
In a medium bowl, whisk together the cream of chicken soup, chicken broth, and sour cream until the mixture is smooth and creamy.
Sprinkle the diced yellow onion evenly over the chicken breasts.
Spread the creamy soup mixture over the chicken, making sure each piece is generously covered.
Place the dry stuffing mix in a separate bowl and drizzle with melted butter. Toss gently until evenly coated, then distribute the stuffing over the chicken mixture.
Cover and cook on LOW for 4–5 hours, or until the chicken reaches an internal temperature of 165°F (74°C). Keep the lid closed as much as possible during cooking.
Allow the casserole to rest for 5–10 minutes before serving. Sprinkle with dried parsley for a finishing touch and enjoy!
