Place the 4–5 lb chuck roast into a 6-quart slow cooker.
Sprinkle ranch seasoning and au jus mix evenly over the roast.
Place butter on top of the roast in small pieces; do not stir.
Add 8–10 pepperoncini around the roast; add optional juice for tang.
Cook on HIGH for 5–6 hours or LOW for 8–9 hours, without lifting the lid.
Check doneness when the roast shreds easily with a fork.
Shred the beef in the slow cooker and skim excess fat if needed.
Stir to coat with gravy, garnish with parsley, and serve warm.