01 -
In a small mixing bowl, whisk together the ketchup, packed brown sugar, apple cider vinegar, Worcestershire sauce, garlic powder, and onion powder until fully combined. Set aside.
02 -
Melt unsalted butter in a small saucepan over medium heat. Stir in all-purpose flour and cook for 1 minute, stirring constantly. Gradually add the whole milk while whisking and bring to a simmer. Simmer for 5 minutes, remove from heat, and blend in shredded cheddar cheese until smooth.
03 -
In a large skillet over medium heat, bring beef broth to a simmer. Gently separate slices of deli roast beef and immerse individually in the hot broth for 1-2 minutes to warm completely. Remove beef from the skillet and reserve.
04 -
Spread prepared Arby's sauce evenly on the bottom half of each onion hamburger bun. Layer with warmed roast beef, spoon cheese sauce generously over meat, and cap with top half of bun. Serve immediately.