Instructions:

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

On a clean, floured surface, roll out the thawed puff pastry into a large rectangle. Cut the pastry into squares or triangles, depending on your preferred shape.

In a mixing bowl, combine the diced ham, Swiss cheese, caramelized onions, and Dijon mustard. Mix well to make sure all ingredients are evenly distributed.

Place a generous spoonful of the ham and cheese filling in the center of each pastry square or triangle. Be careful not to overfill to avoid spilling during baking.

Lightly wet the edges of the pastry with water. Fold the pastry over the filling, pressing down firmly to seal. Use a fork to crimp the edges for a decorative finish.

Beat the egg in a small bowl and use a brush to apply the egg wash to the top of each pastry, avoiding the cut edges. This will give your pastries a beautiful golden color when baked.

Place the prepared pastries on the lined baking sheet and cut a few slits in the top to allow steam to escape.

Bake in the preheated oven for 20-28 minutes, or until the pastries are puffed and golden brown. Remove from the oven and let cool slightly before serving.

Garnish with fresh thyme or parsley before serving for an added touch of flavor and presentation.

For best results, make sure that the puff pastry is thoroughly thawed before rolling it out, as this will help achieve a flaky texture. Additionally, feel free to experiment with different types of cheese or add herbs and spices to the filling for a personalized touch.

Always keep an eye on the pastries while they bake, as oven temperatures can vary, and you want to catch them at their perfect golden moment! Enjoy your delicious creations!