Season the chicken breasts or thighs with salt, pepper, and all additional seasonings if using, and place them in the bottom of the crockpot.
Add cream of chicken soup and chicken broth to crockpot and mix.
If using, sprinkle the diced celery and onion over the top of the chicken.
Sprinkle the stuffing mix over the chicken and broth mixture. Do not stir; just let it sit on top.
Cover the crockpot and cook on low for 6-8 hours or on high for 3-4 hours, until the chicken is cooked through and tender.
Once done, remove the chicken and shred or chop, then add back in and mix everything together. Serve warm and enjoy!