Combine the broth, oregano, basil, garlic powder, red pepper flakes, Worcestershire and pepperoncini brine in a saucepan. Bring to a simmer and cook for 5 minutes to blend the flavors.
Add the cooked roast beef and sautéed bell pepper and sliced pepperoncini. Cover and reheat over medium-low heat, turning the beef, until the center reaches 165°F throughout. Allow approximately 10–15 minutes for refrigerated, sliced or shredded beef; the thermometer reading determines when it is ready.
Serve immediately, or transfer the hot beef and jus to a preheated slow cooker set to warm for holding at 140°F or above.
Split the rolls and toast the cut sides until lightly crisp.
Lift the beef and peppers with tongs, let excess liquid drip back into the pan and fill the rolls. Serve a small cup of the hot jus beside each sandwich if desired.