Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.

Roll out your thawed puff pastry into a large rectangle, about ⅛-inch thick.

Spread a thin layer of Dijon mustard evenly across the puff pastry, leaving a small border.

Sprinkle the mozzarella and Parmesan over the mustard layer and add chopped prosciutto and black pepper.

Fold the pastry over the filling, seal edges tightly, and cut into 1.5-inch squares.

Place the squares on the baking sheet, brush with egg wash, and space them apart.

Bake in the oven for 15-20 minutes until golden brown.

Let cool for 5 minutes on a wire rack before serving.