Instructions:

Preheat the oven to 450°F (230°C). Line a baking sheet with parchment paper.
Roll out one sheet of puff pastry on a floured surface to a 10×12 inch rectangle. Place it on the prepared baking sheet.
Brush the center of the pastry with Dijon mustard, leaving a 1-inch border around the edges.
Layer the ham and cheese over the mustard, still leaving the border.
Roll out the second sheet of puff pastry to the same size and place it over the filling. Press the edges together to seal.
Brush the top with the egg wash and cut a few slits in the top to allow steam to escape.
Bake for 20-25 minutes, until the pastry is puffed and golden brown.
Let it cool for a few minutes before cutting into squares and serving warm.