While the au jus is cooking, lightly toast the bread.
When the au jus is done cooking, add the roast beef and cook in the au jus for 3-5 minutes. (I typically just cook it until the pink is gone, some people like for it to soak longer)
Place the bread on a aluminum covered sheet pan. To one side of the bread, place the au jus cooked roast beef, then the caramelized onions and then the provolone cheese (I typically use two sliced per sandwich).
Place sheet pan in the oven and broil. You will need to watch your sandwiches while they broil because they can burn very easily this way (I have a broiler setting for 500 degrees F. I use that one because it's not as intense). When the cheese is melted and the bread is a deep golden brown, the sandwich is done cooking.
To the top of the sandwich, spread the horseradish sauce.
Pour au jus into bowls for dipping and ENJOY!