Preheat the oven to 400°F (200°C).
In a large mixing bowl, combine chopped spinach, ricotta, mozzarella, Parmesan, garlic powder, and black pepper. Stir until well mixed.
Roll out one sheet of puff pastry on a lightly floured surface to smooth it out.
Spread half of the filling evenly across the rolled-out pastry sheet.
Roll the pastry tightly from one end to the other to form a log shape.
Slice the log into 1-inch thick pieces and place them on a baking sheet lined with parchment paper.
Brush the tops of the pinwheels with the beaten egg for a glossy finish.
Bake in the preheated oven for 15-20 minutes or until golden brown and puffed.
Remove from the oven and let cool slightly before serving.