Roast beef: Preheat your oven to 450 degrees F (233 C).
Combine kosher salt, black pepper, onion powder, garlic powder, MSG, and paprika in a small bowl to make your roast beef rub.
Rub the Dijon mustard on beef and get it coated with a thin layer of mustard all over. Sprinkle the beef with all the rub.
Place the beef on a rack set in a sheet pan and cook at 450 degrees for 20 minutes.
Remove the beef from the oven and reduce the heat to 350 F (175 C). Add the beef back to the oven and cook until the beef temperature hits 130 internal. This should take somewhere between an hour and an hour and a half depending on the size and weight of the beef.
Let the beef cool at least an hour in the refrigerator before slicing (preferably two hours). Slice the beef as thinly as possible.
Cheddar cheese sauce: add both shredded cheeses and half and half to a medium pot over medium heat. Stir to combine. Add mustard powder and paprika.
Cook until cheddar cheese sauce is fully melted and there are no lumps.
Make the sandwich: add sauce to bottom bun, add a small pile of sliced beef on top of the sauce. Drizzle a bit of your cheese sauce on the beef and then cover it all with the top bun. Serve and enjoy.