Heat the olive oil in a large skillet over medium-high heat. Season the beef chuck roast with salt and black pepper, then sear it in the skillet for about 4-5 minutes on each side until browned. Remove the roast and set aside.

In the same skillet, add the sliced onion and cook for 3-4 minutes until softened. Add the minced garlic and cook for an additional 1 minute until fragrant.

In a slow cooker, combine the seared beef, cooked onions, garlic, beef broth, pepperoncini peppers with their juice, Italian seasoning, and red pepper flakes if using.

Cover and cook on low for 8 hours or on high for 4 hours, until the beef is tender and easily shredded.

Once cooked, remove the beef from the slow cooker and shred it with two forks. Return the shredded beef to the slow cooker and stir to combine with the juices.

To serve, fill each hoagie roll with the shredded beef mixture. Top with sliced provolone cheese if desired and garnish with chopped parsley.