Making Spinach and Feta Puff Pastries is easier than you might think. Follow these simple steps for perfect results every time:

1. Prepare the Spinach Filling: Heat the olive oil in a large skillet over medium heat. Add the chopped onion and cook until softened and translucent, about 3-5 minutes. Add the minced garlic and cook for another minute until fragrant, being careful not to burn it.
2. Sauté the Spinach: Add the chopped spinach to the skillet in batches, if necessary. Cook, stirring frequently, until the spinach wilts and reduces in volume. This usually takes about 3-5 minutes. If using frozen spinach, make sure it is thoroughly thawed and squeezed dry before adding it to the skillet. Cook for a minute or two to heat through.
3. Season the Filling: Remove the skillet from the heat. Stir in the crumbled feta cheese, dried oregano, salt, and black pepper (and red pepper flakes, if using). Mix well to combine all ingredients evenly. Taste the filling and adjust seasoning as needed. Remember that feta is salty, so be mindful of the amount of salt you add. Let the filling cool slightly while you prepare the puff pastry.
4. Prepare the Puff Pastry: Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper. Gently unfold the puff pastry sheets on a lightly floured surface. If using frozen puff pastry, ensure it is thawed but still cold. If the pastry is too warm, it will be difficult to work with.
5. Cut the Pastry: Depending on the size and shape you desire, you can cut each puff pastry sheet into squares, rectangles, or triangles. For square pastries, cut each sheet into 9 squares (3×3 grid). For rectangles, cut each sheet into 6 rectangles (2×3 grid). For triangles, cut each square diagonally. You can also use cookie cutters to create different shapes.
6. Fill the Pastries: Place a spoonful of the spinach and feta filling (about 1-2 tablespoons, depending on the size of your pastries) in the center of each pastry square or shape. Be careful not to overfill, as this can make them difficult to seal and cause the filling to leak out during baking.
7. Seal the Pastries: Fold the pastry over the filling to form triangles or rectangles, depending on how you cut them. For triangles, simply fold one corner over to the opposite corner. For rectangles, fold one half over the other. Press the edges firmly to seal, using a fork to crimp the edges for a decorative touch and to ensure they are well sealed. This prevents the filling from escaping during baking.
8. Prepare the Egg Wash: In a small bowl, whisk together the egg and milk (or water) to create an egg wash.
9. Egg Wash and Bake: Brush the top of each pastry with the egg wash. This will give them a beautiful golden-brown color and a glossy finish.
10. Bake: Arrange the filled pastries on the prepared baking sheet, leaving some space between them for even baking. Bake for 20-25 minutes, or until the pastries are golden brown and puffed up. The baking time may vary slightly depending on your oven and the size of your pastries. Keep an eye on them and adjust baking time as needed.
11. Cool and Serve: Once baked, remove the pastries from the oven and let them cool on the baking sheet for a few minutes before transferring them to a wire rack to cool slightly further. This prevents the bottoms from getting soggy. Serve warm or at room temperature.