Preheat the oven to 450 degrees F. Place a piece of parchment paper on a sheet pan.
Lay 1 sheet of puff pastry on a floured board and carefully roll it out to 10 by 12 inches. Place it on a sheet pan and brush the center with the mustard, leaving a 1-inch border around the edge. Place a layer first of ham and then cheese, also leaving a 1-inch border. Brush the border with the egg wash.
Place the second sheet of puff pastry on the floured board and roll it out to 10 by 12inches. Place the second sheet on top of the filled pastry, lining up the edges. Cut the edges straight with a small, sharp knife and press together lightly. Brush the top with egg wash and cut a few slits in the top to allow steam to escape.
Bake for 20 to 25 minutes, until puffed and golden brown. Allow to cool for a few minutes and serve hot or warm.