Place the chuck roast in the bottom of a 6-quart slow cooker.
Scatter the sliced onions and whole pepperoncini peppers over and around the meat.
In a small bowl, whisk together the beef broth, pepperoncini juice, garlic powder, Italian seasoning, red pepper flakes, salt, and black pepper.
Pour the liquid mixture over the beef in the slow cooker.
Cover and cook on LOW for 8 hours, or until the beef easily pulls apart with a fork.
Remove the beef from the slow cooker, shred it using two forks, and return the shredded meat to the juices to soak for 10 minutes before serving.