Instructions:
Prepare the filling:

If using fresh spinach, wash it thoroughly and remove all stems. Heat a large skillet over medium heat and sauté the spinach in olive oil until wilted. If using frozen spinach, drain the spinach’s casings to remove excess moisture.
Once the spinach wilts (or has drained if frozen), place it in a colander and continue to drain. Use a spoon to press out any remaining water.
In the same skillet, sauté the onions and garlic in olive oil until softened and fragrant (about 3 minutes). Remove from heat and set aside to cool.
In a large bowl, combine the spinach, cream cheese, grated mozzarella, Parmesan cheese, sautéed onions, and garlic. Add nutmeg (if using) and season with salt and pepper. Mix well until smooth and creamy.
Assemble the pie:

Preheat your oven to 190°C (375°F).
Roll out one sheet of puff pastry on a lightly floured surface and place it in a lightly greased 9-inch pie plate or tart pan. Press the edges gently to the sides and trim any excess pastry.
Spread the spinach and cheese mixture evenly over the bottom of the pie.
Roll out the second sheet of puff pastry and place it on top of the filling. Press the edges firmly to seal. You can crimp the edges with a fork or your fingers for a decorative border.
Use a sharp knife to cut a few slits in the top of the pastry to allow steam to escape during baking.
Add the egg wash:

Brush the top of the pastry with the egg mixture (beaten egg and water). This will give the cake a beautiful golden color after baking.
Baking:

Place the pie in the preheated oven and bake for 25–30 minutes, until the pastry is golden brown and flaky. You can check the filling by gently lifting the edge of the pastry to ensure it’s piping hot throughout.
Let cool and serve.

Let the pie cool for a few minutes before slicing to allow the filling to set. Serve warm or at room temperature.
Pro tips for the best savory puff pastry pie:
Keep the pastry chilled: Cold puff pastry rises better and forms a beautifully flaky crust. Handle it minimally. If the pastry becomes too soft, chill it briefly before assembling.

Squeeze out excess moisture from the spinach: This prevents the filling from becoming soggy and ensures that your pie stays crispy.

Use fresh mozzarella: Fresh mozzarella gives the filling a wonderfully creamy texture. If you can’t find any, pre-grated mozzarella will also work.

Use baking paper: This makes it much easier to transfer the cake from the work surface to the baking tray (and then to your plate).

Experiment with spices: Add a sprinkle of nutmeg or garlic powder to the filling to give it an extra flavor