In a bowl, mix the diced ham and grated cheese. Add mustard if using and combine well.

Preheat the oven to 200°C (400°F). Roll out the puff pastry on a lightly floured surface to smooth out creases.

Cut the pastry into squares or rectangles. Place a spoonful of the filling in the center of each piece.

Fold the pastry over to create a triangle or rectangle and press the edges to seal. Use a fork to crimp the edges for a decorative touch.

Place the filled pastries on a baking sheet lined with parchment paper. Brush the tops with the beaten egg.

Bake in the preheated oven for 20 minutes or until golden brown.

Remove from the oven and let cool slightly before serving. Enjoy warm!