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

In a medium bowl, combine the chopped spinach, softened cream cheese, mozzarella, Parmesan, minced garlic, salt, and black pepper. Mix until well combined and set aside.

On a lightly floured surface, roll out the thawed puff pastry sheet to smooth it out. Cut the pastry into 3-inch squares.

Spoon about 1 tablespoon of the spinach and cream cheese mixture into the center of each pastry square.

Fold the corners of the pastry over the filling to create a triangle shape and press the edges to seal. Use a fork to crimp the edges for a decorative touch.

Place the prepared puffs on the lined baking sheet. Brush the tops with the beaten egg to give them a golden color while baking.

Bake in the preheated oven for 15-20 minutes, or until the puffs are golden brown and flaky.

Remove from the oven and let them cool slightly before serving. Enjoy your delicious spinach puffs warm or at room temperature.