Preheat oven to 350ºF.
Split the rolls in half and place the bottoms in a large round or 13"x9" dish. On the bottom slices, evenly top in this order: provolone cheese, roast beef, Muenster, (optional sautéed onions and mushrooms), then the bread tops.
In a small bowl, whisk together butter, Worcestershire sauce, Everything Bagel seasoning, and 1 teaspoon of the gravy mix. Pour evenly on top of the sandwiches.
Cover with aluminum foil. Bake 25 minutes. Uncover and bake another 5-10, or until the top of the sandwiches are golden brown and cheese has melted.
While the sandwiches bake, in a small saucepan on medium-high heat, whisk together 2 cups water and the remaining au jus mix. Bring to a boil then lower the heat to simmer 5 minutes.
Serve the sandwiches with the prepared au jus and creamy horseradish sauce on the side.