Place chicken in a slow cooker. Sprinkle au jus mix and ranch seasoning mix on top. Arrange pepperoncini and onion around chicken and cubed butter on top of chicken.
Cook on low until chicken is tender, roughly 8 hours.
Using 2 forks, shred the chicken. Serve over rice and garnish with parsley.
Notes
If you need more liquid in the crockpot, you can add a splash of chicken broth. The recipe sounds like it would be dry, but you simply have to trust the process.
Serve the shredded chicken over mashed potatoes or egg noodles as an alternative to white rice.