Roast beef: Preheat your oven to 450 degrees F (233 C).
Combine kosher salt, black pepper, and MSG (if using) in a small bowl to make your roast beef rub.
Rub the olive oil on beef and get it coated with a thin layer 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.
Garlic slaw: shred your green cabbage until you have about 3 cups worth.
In a large bowl add the cabbage, mayonnaise, garlic, lemon juice, salt, pepper and MSG (if using). Stir everything together until it's fully combined.
Store the slaw in the fridge for up to a week.
Sandwich assembly: Slice a six-inch roll about 2/3rds of the way through and open it up like a book.
Add a mound of thinly sliced roast beef and top that with a slice or two of cheese.
Layer on tomato slices and top that with spoonfuls of garlic slaw.
Close the sandwich and serve.