Prepare the Puff Pastry: Thaw the sheet of puff pastry according to the package instructions, usually about 20-30 minutes at room temperature. Preheat the oven to 200°C (400°F) and line a baking sheet with parchment paper.

Fillings: In a mixing bowl, combine the diced ham and cheese. Add a dash of salt and pepper to taste. If desired, mix in the Dijon mustard for extra flavor.

Assemble the Pastries: Roll out the puff pastry on a lightly floured surface to ensure even thickness. Cut the pastry into squares, approximately 4×4 inches each. Place a generous tablespoon of the ham and cheese mixture into the center of each square. Fold the corners over the filling, pinching to seal, or shape as desired for your preferred style of pastry.

Egg Wash: In a small bowl, whisk together the egg and milk. Brush the tops of the pastries with the egg wash for a golden finish.

Bake: Arrange the pastries on the prepared baking sheet, leaving space between them. Bake in the preheated oven for about 15-20 minutes, or until they are golden brown and cooked through.

Serve: Allow the pastries to cool slightly before serving. Garnish with fresh herbs if desired.