Pat the roast dry. Season all sides with salt and pepper.
Heat olive oil in a large skillet over medium-high. Sear roast until deeply browned, 3–4 minutes per side.
Transfer roast to the slow cooker. Add onion to the skillet; sauté 2 minutes. Stir in garlic for 30 seconds.
Deglaze the skillet with a splash of broth, scraping browned bits. Pour mixture into the slow cooker.
Whisk remaining broth with peperoncini brine, bouillon, Worcestershire, Italian seasoning, oregano, and red pepper flakes.
Pour the mixture over the roast. Scatter sliced peperoncini and onions around the meat.
Cover and cook on Low 8–9 hours, or High 4–5 hours, until the beef is fork-tender.
Shred beef in the cooker with two forks, discarding large fat pieces. Let it soak in the juices 10–15 minutes.
Toast rolls. Pile on beef, top with provolone to melt, and finish with giardiniera. Serve with extra jus for dipping.