Season the beef with the salt and pepper and sear it in the oil in a hot skillet until browned on two sides, 5 minutes per side.
Place the onion in a 6-quart slow cooker and set the beef on top.
Add the pepperoncini, reserved brine, broth, oregano and garlic powder.
Cover and cook on low until the beef shreds without resistance, 7 1/2 to 8 hours.
Transfer the beef to a board, discard large fat pockets and shred the meat into bite-size pieces.
Return the beef to the pot liquid for 10 minutes while toasting the rolls.
Fill each roll with drained beef and pepperoncini, top with provolone and broil just until the cheese melts.
Serve a small bowl of strained cooking liquid alongside each sandwich for dipping.