01 - Preheat the oven to 390°F. Line a baking tray with parchment paper.
02 - In a large skillet over medium heat, sauté the onion in a splash of olive oil until translucent, about 3 minutes. Add garlic and cook for 1 minute until fragrant.
03 - Add the spinach and cook until wilted and most moisture has evaporated. Season lightly with salt, pepper, and nutmeg. Let cool for 5 minutes.
04 - In a bowl, combine the cooled spinach mixture with crumbled feta, ricotta or cream cheese, chopped herbs, and 1 beaten egg. Mix until well combined.
05 - Unroll the puff pastry onto the prepared tray. Spoon the filling down the center lengthwise, leaving a 1.5 inch border along the edges.
06 - Fold the sides of the pastry over the filling to enclose, pinching or tucking to seal. Flip seam-side down.
07 - Brush the top of the pastry with beaten egg yolk for a golden finish.
08 - With a sharp knife, make a few small slits on top to allow steam to escape during baking.
09 - Bake for 25 minutes or until golden brown and crisp.
10 - Let cool slightly before slicing and serving. Enjoy warm or at room temperature.