01 -
In a small bowl, whisk together ketchup, brown sugar, apple cider vinegar, Worcestershire sauce, garlic powder, and onion powder until well combined. Set aside.
02 -
In a small saucepan over medium heat, melt the butter. Once melted, stir in the flour and cook for 1 minute, stirring constantly to create a roux.
03 -
Slowly pour in the milk while whisking constantly to prevent lumps. Bring to a simmer and cook for 5 minutes until thickened. Remove from heat and stir in the shredded cheese until melted and smooth.
04 -
In a large skillet over medium heat, bring the beef broth to a gentle simmer for about 5 minutes to heat through.
05 -
Separate the roast beef slices and add them to the simmering broth one by one to prevent sticking. Let them warm for just 1-2 minutes - you don't want to cook them, just heat and flavor them.
06 -
Split the onion buns and spread the homemade Arby's sauce generously on the bottom half of each bun.
07 -
Using tongs, remove the warmed roast beef from the broth and pile it generously on each sauced bun bottom. Drizzle the warm cheese sauce over the beef.
08 -
Top with the other bun half and serve immediately while the cheese sauce is still warm and gooey. Enjoy your homemade version of this iconic sandwich!