Preheat the oven to 400°.  Line a large baking sheet with a piece of parchment.

Lay 1 sheet of puff pastry on a floured counter and roll it out to a 10 x 12-inch rectangle.  Place it on the prepared sheet pan and brush with Dijon mustard, leaving a 1-inch border.  Sprinkle the chives over the mustard to help them stick.  Place a layer of ham and a layer of cheese, still leaving a 1-inch border. Brush the unfilled border with the egg wash.  Place the second sheet of puff pastry on the floured counter and roll it out to a 10 x 12-inch rectangle.  Place the second sheet on top of the pastry and fillings, lining up the edges and pressing together lightly to seal.  You can use a sharp knife to cut the edges of the puff pastry to make straight edges, if desired.  Brush the top with egg wash and cut a few slits in the top pastry to allow steam to escape.  Sprinkle the poppy seeds over top.

Bake for 30 to 35 minutes, until puffed and golden brown.  Allow to cool for at least 5 minutes before slicing.