Saute Mushrooms until golden and soft. Sprinkle with garlic salt and pepper to taste. Remove mushrooms from pan and set aside. In the same skillet (without cleaning it), add 1 1/2 cups beef broth and boil for a few minutes to reduce slightly. Season broth to taste (you want it to be flavorful for use as a dip) then remove from heat - this broth is your "au jus" dipping sauce.
Heat a large pan over medium heat. Add a small pat of butter and when butter is done sizzling, Saute the cut side of the rolls just until golden. Divide the pastrami meat evenly among the four sandwiches. Top with cheese and mushrooms.
Bake at 450 for 4-5 minutes or until cheese melts and meat is hot.
Serve each sandwich with an individual ramekin of AuJus – Dip with every bite.