Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper. This helps prevent sticking and allows the pastry to bake evenly. Gather all ingredients before starting for a smooth cooking process.

Place the puff pastry on a lightly floured surface. Roll it gently if needed. Cut the pastry into twelve equal squares. Try to keep the sizes even for neat and uniform stacks.

Place six pastry squares on the prepared baking sheet. Spread a small amount of Dijon mustard on each square. Add one slice of ham and one slice of cheese on top of each piece.

Cover each filled pastry square with another pastry square. Press the edges gently using a fork. This helps seal the filling inside and creates a decorative edge around each stack.

In a small bowl, whisk together the egg and water. Brush the mixture over each pastry stack. Sprinkle dried parsley and Parmesan cheese evenly across the tops for extra flavor.

Bake for about 18 to 20 minutes. The pastry should become golden brown and puffed. The cheese inside should be melted and warm. Keep an eye on them during the final minutes.

Remove the stacks from the oven. Let them cool for about five minutes. This helps the cheese settle slightly. Serve warm and enjoy the crispy layers and savory filling.