Prepare the Egg WashIn a small bowl, whisk together the egg and water. Set aside.
Mix the FillingIn another bowl, combine the crumbled feta, grated mozzarella, chopped green onion, garlic salt, and nutmeg.
Roll Out the Puff PastryOn a lightly floured surface, unfold the thawed puff pastry and roll it out slightly to smooth any creases.
Assemble the SpiralsBrush the entire surface of the pastry with the egg wash. Evenly spread the cheese mixture over it, leaving a 1-inch border on one short side. Scatter the drained spinach evenly on top.
Roll and SliceStarting from the opposite short side, tightly roll the pastry into a log. Press the edge to seal. Using a sharp knife, cut the log into ½-inch slices.
Chill Before BakingArrange the slices on a parchment-lined baking sheet and refrigerate for about 15 minutes to help them hold their shape.
Bake to Golden PerfectionPreheat the oven to 400°F (200°C). Brush the tops of the pinwheels with the remaining egg wash. Bake for 15 minutes, or until golden brown and puffed.
Serve and EnjoyLet them cool slightly before serving. Enjoy warm or at room temperature.