Preheat oven to 400°F (200°C). Line a large baking sheet with parchment paper. Lightly flour your work surface and gently roll out both puff pastry sheets to smooth creases.
Layer the filling. Place one pastry sheet on the prepared baking sheet. Arrange ham slices evenly across the surface, then top with cheese slices. Spread Dijon mustard over the cheese if using.
Seal the pastry. Place the second puff pastry sheet on top. Press gently around edges to seal, then crimp with a fork. Cut 3-4 small slits across the top for steam vents.
Apply egg wash. Brush the entire surface with beaten egg for a golden-brown finish.
Bake until golden. Bake in center of oven for 20-25 minutes until puffed and deeply golden. Tent with foil if browning too quickly.
Rest and serve. Let cool for 5 minutes before slicing into squares or rectangles. Serve warm for best texture.