In a medium bowl, mix chopped spinach, mozzarella, ricotta, parmesan, garlic, and egg. Season with salt, pepper, and optional nutmeg or chili flakes. Stir until well combined and chill while preparing dough.

In a large bowl, whisk flour, salt, and baking powder. Add cold butter and work into the flour with fingers or pastry cutter until crumbly. Add the egg and 3–5 tablespoons of cold water, mixing until a soft dough forms. Wrap in plastic and chill for 30 minutes.

Preheat oven to 375°F (190°C) and line a baking sheet with parchment. Roll out the dough to 1/8 inch thick between two sheets of parchment. Cut into 3-inch squares or circles.

Place 1 heaping tablespoon of filling in the center of each dough piece. Fold over and crimp edges with a fork to seal.

Arrange puffs on the baking sheet. Brush with beaten egg. Bake for 20–25 minutes or until golden and puffed.

Cool slightly before serving. Store leftovers in the fridge for 3 days or freeze unbaked for future baking.