First, get your oven ready by preheating it to 400°F (200°C). While the oven heats up, lay out your thawed puff pastry sheet on a lightly floured surface. This preparation step ensures your kitchen is ready for a smooth and efficient cooking process, allowing the puffs to bake evenly.
In a medium-sized mixing bowl, it's time to create the heart of the dish. Combine the softened cream cheese, chopped fresh spinach, grated parmesan cheese, garlic powder, and onion powder. Mix everything together until it's well combined, then season with salt and pepper to your liking.
Now, gently roll out the puff pastry sheet just a bit to smooth any creases. Cut the sheet into uniform squares, about 3x3 inches each. Place a generous spoonful of the spinach and cream cheese mixture into the center of each pastry square. Be careful not to overfill them.
Take one corner of a pastry square and fold it over to the opposite corner to form a triangle. Press the edges together firmly to seal the filling inside. For an extra secure seal and a decorative touch, you can use the tines of a fork to crimp the edges.
Arrange the filled puffs on a baking sheet lined with parchment paper. Brush the tops of each one with the beaten egg. This egg wash is the secret to a shiny, golden-brown finish. Bake for 15-20 minutes, or until they are puffed up and beautifully golden. Let them cool for a few minutes before serving.