Preheat the oven to 400°F.
Thaw the puff pastry on a lightly floured surface for about 30 minutes.
In a large mixing bowl, combine the shredded roast beef, Swiss cheese, provolone cheese, Dijon mustard, garlic powder, and black pepper.
Roll out one sheet of puff pastry on a floured surface to fit a 9x13 inch baking dish.
Press the puff pastry into the baking dish, allowing excess pastry to hang over the edges.
Spread the beef and cheese mixture evenly over the puff pastry.
Fold the excess pastry over the filling to enclose it, sealing the edges with a fork.
Brush the top with an egg wash made from 1 beaten egg and 1 tablespoon of water.
Bake in the preheated oven for 25-30 minutes, or until golden brown.
While baking, heat the beef broth in a small saucepan.
Remove the squares from the oven and let them cool for a few minutes before slicing.