In a non-stick skillet, add the heavy cream and shredded cheddar. Heat over medium-low, stirring constantly until the cheese has melted and is smooth. Cover and set aside.
For the sandwich:
Warm the beef stock in a 2-quart or larger saucepot.
Add the roast beef to the beef stock 3-4 pieces at a time and heat for 30-60 seconds. Remove using tongs and transfer to a plate. Repeat with the rest of the roast beef.
To assemble the sandwiches, slice the Kaiser rolls if necessary, spread 1 tablespoon of Arby's sauce on the bottom bun, add ¼ of the roast beef, drizzle on 3-4 tablespoons of the cheese sauce, add the top bun, and enjoy!
Notes
If your cheese sauce solidifies, warm it over low heat and add 1-2 tablespoons of heavy cream, stirring until re-melted and smooth.
I recommend shredding your own cheese for the cheese sauce, as it melts better than pre-packaged shredded cheese.