Preheat oven to 275°F (135°C). Pat beef dry with paper towels. Rub with oil, then season with salt, pepper, and garlic powder.
Heat a large oven-safe skillet over high heat. Sear roast on all sides until deeply browned, about 2 minutes per side.
Add beef broth to skillet. Bring to a simmer, then cover tightly with foil or a lid. Transfer to oven and roast for 2-2.5 hours until fork-tender.
Remove roast from liquid and let rest 10 minutes. Reserve cooking liquid for au jus if desired.
Thinly slice beef against the grain into very thin strips (use a sharp knife or deli slicer).
For each sandwich: split a roll and warm in a dry skillet or oven. Pile on sliced beef, add a slice of cheese if using, and drizzle with Arby's sauce. Serve with extra sauce and au jus on the side.
Tips
For authentic texture, slice beef as thin as possible; chilling the roast for 30 minutes before slicing helps.
Leftover beef and sauce can be stored separately in the fridge for up to 3 days.
To make Arby's sauce: whisk all sauce ingredients together and let sit 30 minutes for flavors to meld.