Arrange the onions and garlic in the bottom of a slow cooker. Set aside.
Pat beef dry and season with kosher salt and black pepper on all sides.
Heat oil in a large pot or Dutch oven over medium-high heat. Place beef in the pot and sear for about 5-7 minutes on each side, or until browned all over. Transfer beef to the slow cooker (on top of the onions and garlic).
Pour the beef broth into the Dutch oven; turn the heat down to medium. Using a wooden spoon, scrape up any browned bits from the bottom of the pot. These bits add a lot of flavor to the broth!
Pour beef broth over the roast. Add Worcestershire sauce and sprinkle with dry Italian salad dressing mix. Add pepperoncini to the slow cooker, along with 1 tablespoon of liquid from the jar.
Cover and cook on LOW for 8-10 hours or on HIGH for 4-5 hours.
Shred the beef with two forks (it should practically fall apart when it’s done). Discard any big fatty pieces. Stir the shredded beef into the juices in the pot.
To assemble sandwiches: split the hoagie rolls in half and toast lightly in a 350°F for 5 to 7 minutes. Add beef to each roll and top with provolone. Return to the oven just until cheese melts. Top with giardiniera and serve!