Spray the crock of a 6-quart slow cooker with nonstick spray. Set aside.
Heat the butter in a skillet set over medium heat.
2 tablespoons unsalted butter
Sauté onion and celery for 5 minutes, until softened and translucent, then transfer to a large bowl to cool.
½ cup diced onion, ½ cup diced celery
Meanwhile, season the chicken with Italian seasoning, garlic powder, salt, and pepper, and place it in the bottom of the slow cooker.
1½-2 pounds boneless, skinless chicken breasts, 1 tablespoon Italian seasoning, 2 teaspoons garlic powder, Kosher salt, Freshly ground black pepper
Sprinkle the stuffing mix over top of the chicken.
6 ounces chicken stuffing mix
To the bowl of vegetables, add the soup, sour cream, and chicken broth. Stir well to combine, then pour the mixture over the chicken and stuffing.
10.5 ounces cream of chicken soup, 8 ounces sour cream, ½ cup low-sodium chicken broth
Cook on high for 4 hours, or low for 6-7 hours.