Creating these delicious pastries is straightforward and quick. Follow these steps to ensure perfect results every time:

1. Preheat your oven to 400°F (200°C).
   Ensuring your oven is preheated is crucial for achieving that perfect, golden-brown finish on your puff pastry.
2. Roll out the puff pastry on a floured surface and cut into squares (about 4×4 inches).
   Using a sharp knife or a pastry cutter, make sure the squares are even to ensure consistent cooking.
3. Place 1 tablespoon of pesto and a generous amount of mozzarella cheese in the center of each square.
   Be careful not to overfill the squares to prevent overflow during baking.
4. Fold the pastry over to create a triangle and seal the edges by pressing with a fork.
   This step is crucial to ensure the fillings stay inside the pastry during baking.
5. Brush the tops with the beaten egg and sprinkle with salt and pepper.
   The egg wash gives the pastry a beautiful sheen and helps the salt and pepper adhere to the surface.
6. Bake for 15-20 minutes or until golden brown.
   Keep an eye on them as they bake to avoid overcooking.