01 - Scatter the thinly sliced onion and minced garlic evenly across the bottom of the slow cooker insert.
02 - Combine the kosher salt, black pepper, Italian seasoning, dried basil, and oregano in a small bowl. Rub the seasoning mixture over all sides of the chuck roast, pressing gently to adhere.
03 - Place the seasoned roast on top of the onion and garlic bed in the slow cooker.
04 - Pour the beef broth and pepperoncini juice over and around the roast. Sprinkle with crushed red pepper flakes if using.
05 - Cover the slow cooker and cook on LOW for 8 hours or on HIGH for 5–6 hours, until the beef is fall-apart tender and shreds easily with a fork.
06 - Transfer the roast to a large cutting board. Using two forks, shred the beef, discarding any large pieces of excess fat. Return the shredded meat to the slow cooker and stir to combine with the cooking juices.
07 - If desired, spread softened butter on the cut sides of the sandwich rolls. Place under the broiler on a baking sheet until golden brown, about 1–2 minutes.
08 - Mound the shredded beef generously onto each toasted roll. Layer with 2 slices of provolone cheese and a handful of sliced pepperoncini.
09 - For a golden, melty finish, place the assembled open-faced sandwiches under the broiler for 1–2 minutes until the cheese is bubbling. Serve warm with a side of au jus for dipping if desired.