To Caramelize Onions--set a saute pan over medium low heat, add olive oil and cook onions, seasoned with salt and pepper, slowly for 20 minutes or more, stirring occasionally until they turn caramel brown in color.
When caramelized, add stock, allow to simmer and reduce, about 10 minutes.
When stock is reduced a bit, lay sliced roast beef over the onions and stock and allow to warm through. Turn oven broiler on high.
STEAK OPTION--While the onions are caramelizing, you can alternatively cook a steak on an outdoor grill, allow it to rest on a cutting board before slicing and adding to the reduced stock. See Method 2 and 3 in text above.
Assemble sandwiches on a large sheet tray with tongs, fill hoagies with roast beef, onions, and top with cheese. Broil until cheese is melted and hoagies are toasted. Meanwhile turn up heat to medium to reduce the beef stock and onion mixture further, this will be your dipping sauce.
Cut sandwiches diagonally to allow for easy dipping! Serve everyone their own little cup of sauce.