Spray the inside of a 4-6 quart slow cooker with non-stick cooking spray.
Place the chicken breasts in the bottom of the slow cooker in a single layer.
In a medium bowl, combine the dry stuffing mix, diced onion, diced celery, frozen mixed vegetables, dried thyme, garlic powder, black pepper, and salt.
In a separate small bowl, whisk together the condensed cream of chicken soup, chicken broth, and melted butter until smooth.
Pour the soup mixture evenly over the chicken breasts in the slow cooker.
Sprinkle the stuffing mixture evenly over the top of the chicken and soup mixture, spreading it out to cover the surface.
Cover the slow cooker with the lid and cook on low for 4 to 4 1/2 hours, or until the chicken is cooked through and the stuffing is tender.
Carefully remove the chicken breasts and fluff the stuffing with a fork before serving.
Serve the chicken breasts with a generous scoop of the stuffing and vegetables.