Preheat your oven to 400°F (200°C) and prepare a baking tray lined with parchment paper.

Roll out the chilled puff pastry on a floured surface to about 1/8 inch thick and cut into 4x6 inch rectangles.

On one half of each pastry rectangle, layer thinly sliced ham and a generous amount of shredded cheese. Optionally, spread a thin layer of Dijon mustard on the ham.

Fold the pastry over to encase the filling and pinch the edges firmly to seal.

Whisk an egg to create an egg wash and gently coat the tops of each pastry with the egg mixture.

Place the pastries on the prepared baking tray and bake for 20–25 minutes until golden brown and crispy.

Let the pastries cool on a wire rack for about 5 minutes before serving warm.