Cup of Yum
1. Heat the olive oil in a large skillet over medium heat. Add the onion and garlic and cook until just softened (be careful not to burn the garlic). Add the spinach and cook, stirring often, until fully wilted, about 3 minutes.
2. Remove from the heat. Season lightly with salt and pepper and vigorously stir in the lemon juice and egg yolk (reserve the egg white for later). Transfer the mixture to a bowl and allow to cool.
3. Once the spinach is cool enough to handle, squeeze out as much liquid as possible and roughly chop. (I realize that it seems counter-intuitive to put liquid in then squeeze it out, but after countless test runs this process led to the best results.) Return spinach to the bowl and thoroughly mix in the feta.
4. Preheat the oven to 400°F (200ºC). For a convection oven, reduce cooking temperature by 25ºF (15ºC). Line a baking sheet with parchment paper.
5. Roll out the puff pastry to a rectangle about ⅛ inch (3 mm) thick and 12" x 18" (30 x 46 cm). Cut into 6 even 5"-6" (13-15 cm) squares. Put a spoonful of the spinach-feta mixture into the center of each puff pastry square. Fold the puff pastry over to form a triangle. Press down with a fork along the edges to tightly seal.
6. Transfer the turnovers to the prepared baking sheet. Brush with the reserved egg white and sprinkle with za’atar, if using. Put in the oven and bake for 20 minutes, or until golden brown.
7. Remove from the oven and allow to cool slightly before serving. Spinach-feta turnovers are excellent warm or at room temperature.

To Store

1. The turnovers can be made ahead and frozen. To freeze, lay flat on a baking tray and put in the freezer. Once frozen, transfer to freezer bags. To re-heat, simply bake in a pre-heated oven for 10 to 15 minutes, until warmed through.