Place the chuck roast into the slow cooker.
In a small bowl, mix the salt, ground pepper, garlic, and dry mustard.
Sprinkle the seasoning on the roast in the slow cooker.
Add the ranch dressing, the whole envelope, and the Au Jus gravy envelope.
Cut the butter into pieces and place on top of the roast.
Add the pepperoncinis and place them on the roast.
Add pepperonici juice and beef broth.
Cook on high for 5 hours or low for 8 hours.
Once the roast is tender. Carefully take the roast out of the slow cooker into a large bowl, and the pepperoncini out in a separate bowl. With 2 forks shred the beef into pieces.
Place the beef back into the slow cooker and mix it with the sauce.
Leave in the slow cooker until ready to serve.
Serve over mashed potatoes!