Preheat your oven to 400°F (200°C).
In a mixing bowl, combine the shredded Muenster cheese, grated Parmesan, garlic powder, and thawed, well-drained spinach. Mix until evenly distributed.
Lightly flour your work surface and roll out the thawed puff pastry sheet to smooth out any creases.
Spread the spinach and cheese mixture evenly over the puff pastry, leaving a small border around the edges. Starting from one edge, carefully roll the pastry tightly into a log shape.
Using a sharp knife or pizza cutter, slice your rolled pastry into 1-inch thick pieces. You should have about 12-15 pinwheels.
Place the pinwheels on a baking sheet lined with parchment paper. Whisk together the egg and water to create the egg wash. Brush the tops of the pinwheels with the egg wash.
Bake the pinwheels in the preheated oven for 15-20 minutes, or until they are puffed and golden brown.
Once baked, remove the pinwheels from the oven and allow them to cool slightly on the baking sheet. Serve warm or at room temperature.
