Makes 8 turnovers
1 1/2 tablespoons extra-virgin olive oil
1 1/2 onions, finely chopped
3 boxes (9 ounces each) frozen chopped spinach, thawed and squeezed dry of excess water
1 1/2 cup (about 12 ounces) crumbled feta
2 tablespoons fresh lemon juice
1/4 teaspoon cayenne pepper
Coarse salt and freshly ground pepper
All-purpose flour, for dusting
1 17-ounce box puff pastry, thawed
1 large egg, beaten
2 tablespoons water
Heat oil in a skillet over medium-range heat. Add onions and garlic, cooking until tender, about 5 to 7 minutes. Transfer the cooked onions and garlic to a large bowl; mix in the spinach, feta, lemon juice and cayenne pepper. Set aside.
Preheat oven to 375 degrees. On a lightly floured surface, roll out each sheet of puff pastry dough to a 12-inch square. Cut each sheet into quarters (four even sections) to form a total of eight 6-inch squares from the two sheets of pastry.
Evenly distribute the spinach-feta mixture among the eight squares of puff pastry. Spoon filling onto center of each square. Beat egg and water; lightly brushing 2 edges of each square with the egg wash to form a glue/bond. Fold the edges over the filling to form a triangle. Press firmly to seal. With a fork, crimp edges and ensure that the dough edges are tightly sealed around the spinach-feta filling.
Transfer the turnovers to parchment-lined rimmed baking sheets. Brush tops with remaining egg wash. Bake until golden and the pastry is puffed, about 35 to 40 minutes.
Serve warm.
Planning Ahead: These turnovers can be made ahead and frozen up to two months in advance. Freeze turnovers individually on a parchment-lined baking sheet. Wrap frozen turnovers in plastic wrap and secure in a sealable freezer bag.
To Bake When Frozen: Bake in a preheated 375 degree oven for about 45-50 minutes.