Place the thawed spinach in a clean kitchen towel or several layers of paper towels. Squeeze firmly until no more liquid drips out. Separate any compacted clumps with your fingers.

In a medium bowl, stir the softened cream cheese until smooth. Add the spinach, mozzarella, Parmesan, garlic, Italian seasoning, salt, and black pepper. Mix until evenly combined. The filling should be thick and spreadable, with no visible liquid in the bowl.

Unfold the thawed puff pastry on a lightly floured surface. Roll it gently into a rectangle approximately 10 x 12 inches (25 x 30 cm), smoothing the fold lines without pressing out the pastry layers.

Spread the spinach mixture evenly over the pastry, leaving a 1/2-inch (1.25 cm) border along all four sides. Keep the filling in a thin, uniform layer so the roll remains easy to slice.

Starting with one long side, roll the pastry into a tight log. Keep the roll snug without crushing the layers. Pinch the final edge lightly against the log to seal it.

Place the pastry log seam-side down on a sheet of parchment paper. Wrap it loosely and refrigerate for 30 minutes. It should feel firm enough to cut without flattening.

Preheat the oven to 400°F (200°C). Line a large rimmed baking sheet with parchment paper.

Whisk the egg and water together in a small bowl until fully blended.

Trim the uneven ends from the chilled pastry log. Cut the log into 20 slices, each approximately 1/2 inch (1.25 cm) thick. Use a gentle sawing motion rather than pressing straight down.

Place the pinwheels cut-side up on the prepared baking sheet, leaving about 2 inches (5 cm) between them. Reshape any flattened spirals gently with your fingers.

Lightly brush the exposed pastry edges and tops with the egg wash. Avoid flooding the cut sides, as excess egg can seal the pastry layers and limit their rise.

Bake on the center rack for 18 to 22 minutes. The pastry should be puffed and deep golden, and the cheese should be melted with light browning around the edges.

Lift one pinwheel carefully with a spatula: The bottom should look crisp and golden rather than pale or damp. If needed, bake for another 2 minutes.

Transfer the baking sheet to a wire rack and let the pinwheels cool for 10 minutes. The filling will be very hot and will firm slightly as it rests.

Sprinkle with chopped parsley, if using: Serve warm or at room temperature.