In a large saute pan, melt 2 tablespoons of butter and over medium heat, cook onions until mostly caramelized (don't let them get dry or begin to burn, you want them moist).

Heat oven to 350 degrees.

Once the onions are done to your liking, add in the beef broth and Worcestershire. Season with black pepper, onion powder, and garlic powder. Add salt if you feel its necessary since the stock/broth may be salty. (You can also switch from onion or garlic powder to onion or garlic salt...just watch out for over-salting).

Bring mixture just to a simmer and then turn off the heat.

Arrange your rolls in a casserole dish, nestling them together. Either remove the tops from the rolls or "open them" leaving one side intact.

If you like, brush the bottom roll with just a bit of mayonnaise, top that with a bit of the shredded Gruyere, then top that with a couple of slices or so of roast beef, folded.

Top Roast beef with more of the Gruyere cheese and top the cheese with the onions (using a fork or tongs to remove onions from the liquid).

Close the rolls or place the tops of the rolls back on, and brush rolls all across the top with some of the remaining liquid from the pan. Sprinkle tops with a pinch or two of onion powder and the Thyme leaves and bake for 15-20 minutes or until sandwiches are heated through and cheese is melted.

If you enjoy dipping your sandwiches in an au jus, use the remaining liquid for dipping (and/or double it if you like).