Heat oven to 425 degrees.
Cut a piece of parchment paper to fit a standard baking sheet; set aside. Prepare egg wash; set aside. In a small bowl, combine the Mustard Sauce ingredients; set aside.
Lay a sheet of thawed pastry onto a lightly floured surface and use a rolling pin to roll it to about 10"X12" then transfer to the parchment lined sheet.
Spread Mustard Sauce onto the layer of pastry, leaving 1/2" around the border. Place ham, then cheese, careful not to intersect any ingredients with the 1/2" border.
Brush egg wash all around the 1/2" border.
Roll out the second layer of pastry to the same dimensions, then place it over the other pastry. Align edges (if it's not perfect, that's ok - you can just trim the edges if needed) and press layers together to ensure a seal with the egg wash. Press the tines of a fork into the layers to form a stronger seal all around.
Brush the egg wash over the entire layer of the puff pastry (including the edges, to ensure even browning) and cut several slits in the top for steam to escape. Bake about 25 minutes or until it's golden brown.
Remove from oven and allow to cool at least 15-20 minutes before serving.