Spray the crock of a 6-quart slow cooker with nonstick spray. Set aside.: The air should smell clean with a faint chemical note from the pan spray, and the coated crock prevents sticking so the stuffing does not glue to the sides. This small step helps ensure easy serving and cleanup, which is worth the extra minute. A common mistake is skipping this, which can make the stuffing cling and tear when you try to scoop it out.
Heat the butter in a skillet set over medium heat.: You will hear a gentle sizzle when the butter melts, and it should shimmer and smell slightly nutty without darkening. This warm fat is the flavor carrier for the aromatics, allowing them to release their sweetness and soften evenly. If the butter browns too fast, reduce the heat, because burnt butter will introduce bitter notes to the dish.
Sauté onion and celery for 5 minutes, until softened and translucent, then transfer to a large bowl to cool.: As the onion and celery cook, their color will change to translucent, and their scent will become sweet and savory. Cooling them briefly prevents the heat from cooking other ingredients prematurely when combined. Avoid rushing this step, because undercooked vegetables will leave a raw, sharp bite instead of melding into the creamy sauce.
Meanwhile, season the chicken with Italian seasoning, garlic powder, salt, and pepper, and place it in the bottom of the slow cooker.: The dried herbs and spices should coat the chicken evenly, imparting an aromatic crust as it begins to cook. You might notice the fragrant herbs and a faint garlic scent as the seasoning settles. A typical error is under seasoning, which leads to a bland center, so be sure to distribute the seasonings uniformly.
Sprinkle the stuffing mix over top of the chicken.: The dry stuffing will sit like a blanket over the seasoned chicken , and its texture will be noticeably dry and crumbly before cooking. It acts as a sponge for the creamy mixture that follows, absorbing liquid to become tender. If you press it down too densely, it may not hydrate evenly, so scatter it rather than pack it tightly.
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.: The sauce should be smooth and pale, with a rich, creamy aroma from the condensed soup and sour cream. Pouring it gently over the stuffing lets it seep in gradually, ensuring even hydration. Stirring until homogeneous prevents streaks of sour cream or soup, and a common slip is pouring without mixing, which can leave clumps in the finished dish.
Cook on high for 4 hours, or low for 6-7 hours.: During cooking you will notice the aroma deepen, a gentle bubbling at the edges, and the chicken turning opaque and tender. The stuffing will swell as it absorbs the sauce, and the surface may take on a slightly darker, set appearance where it contacts the crock. Resist the urge to lift the lid frequently, because losing heat prolongs cooking time and can lead to uneven texture.