Preheat your oven to 400°F and line two baking sheets with parchment paper.
In a medium bowl, mix together the squeezed-dry spinach, cream cheese, mozzarella, Parmesan, garlic, nutmeg, salt, and pepper until well combined.
Roll out the puff pastry sheets on a lightly floured surface and cut each sheet into 12 squares (about 3x3 inches each).
Place about 1 tablespoon of the spinach mixture in the center of each square.
Fold the corners up to meet in the middle, pinching the edges together to seal (they'll look like little purses).
Place the bites seam-side down on your prepared baking sheets, spacing them about 2 inches apart.
Brush the tops with beaten egg for that beautiful golden color.
Bake for 16-18 minutes until puffed and golden brown. Let them cool for 5 minutes before serving warm.