In a mixing bowl, combine the shredded Muenster cheese, grated Parmesan, garlic powder, and the thoroughly dried spinach. Mix until well combined.
On a clean, lightly floured surface, roll out the thawed puff pastry sheet to an even thickness of about 1/8 inch.
Spread the cheese and spinach mixture evenly over the puff pastry, leaving a small border. Roll the pastry into a tight log and slice into 1-inch thick pinwheels.
In a small bowl, whisk together the egg and water to create the egg wash.
Preheat your oven to 400°F (200°C).
Line a baking sheet with parchment paper. Arrange the pinwheels, brush with egg wash, and sprinkle extra Parmesan if desired. Bake for 15-20 minutes until golden brown.
Let the pinwheels cool for a few minutes before serving. Best enjoyed warm or at room temperature.