Trim any visible fat from the chuck roast. Cut the roast into 2–3 large pieces so it cooks more evenly.
Add the sliced onion and bell peppers to the bottom of a slow cooker. Place the beef on top of the vegetables.
Whisk together the beef broth, pepperoncini juice, Worcestershire sauce, Italian seasoning, garlic powder, onion powder, oregano, black pepper, salt, and red pepper flakes.
Pour the mixture over the beef and add the pepperoncini peppers.
Cover and cook on low for 7–8 hours, or until the beef is very tender and easily pulls apart with a fork.
Remove the beef and shred it with two forks. Return it to the slow cooker and stir it into the peppers and flavorful juices.