Follow these detailed, step-by-step instructions for perfect, flaky, and flavorful spinach puff pastries every time. The process is straightforward and can be broken down into preparing the filling and then assembling and baking the pastries.

Part 1: Preparing the Savory Spinach and Cheese Filling

1. Prepare the Spinach: The most crucial step for avoiding soggy pastries is to prepare your spinach correctly. Place the thawed frozen spinach in the center of a clean kitchen towel, cheesecloth, or several layers of heavy-duty paper towels. Gather the ends and twist, squeezing as hard as you can over a sink or bowl to remove as much liquid as possible. You’ll be surprised at how much water comes out. The resulting spinach should be a compact, dry ball. Set it aside.
2. Sauté the Aromatics: In a medium skillet, heat a tablespoon of olive oil or butter over medium heat. Add the finely minced yellow onion and sauté for 4-5 minutes, or until it becomes soft and translucent. Add the minced garlic and cook for another minute until fragrant. Be careful not to let the garlic burn. Remove the skillet from the heat and let the onion and garlic mixture cool for a few minutes.
3. Combine the Filling Ingredients: In a large mixing bowl, combine the softened cream cheese and the crumbled feta cheese. Use a spatula or a fork to mix them together until they are relatively smooth.
4. Add Remaining Components: Add the squeezed, dry spinach to the cheese mixture. Add the cooled onion and garlic mixture, the lemon zest, dried dill, and black pepper.
5. Incorporate the Egg: In a small, separate bowl, whisk the large egg. Pour about three-quarters of the whisked egg into the spinach and cheese mixture, reserving the remaining one-quarter in the small bowl for the egg wash later.
6. Mix and Season: Gently fold all the ingredients together until everything is well combined. Now is the time to taste the filling. It should be savory and tangy. If needed, add a small pinch of salt, but remember the feta is already salty. The filling should be thick and not watery. If it seems too wet, you may not have squeezed enough water from the spinach.

Part 2: Assembling and Baking the Puff Pastries

1. Preheat and Prepare: Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper. This prevents the pastries from sticking and makes cleanup a breeze.
2. Prepare the Pastry: Gently unfold your thawed puff pastry sheets on a lightly floured surface. If the sheets are thick, you can use a rolling pin to lightly roll them out just a bit larger, to about a 10×10 inch square. This helps even out the thickness.
3. Cut the Pastry: Using a sharp knife or a pizza cutter, cut each sheet of puff pastry into 9 equal squares (for a total of 18 pastries).
4. Fill the Pastries: Place a heaping tablespoon of the spinach and cheese filling onto the center of each pastry square. Be careful not to overfill, as this can cause the pastries to burst open while baking. An overfilled pastry will not puff up as nicely.
5. Fold and Seal: To form the pastries, you can choose your favorite shape.
   • For Triangles: Gently fold one corner of the square over the filling to meet the opposite corner, forming a triangle.
   • For Rectangles: Fold one side of the square over the filling to meet the other side, forming a rectangle.
     Press the edges firmly together to seal. For a more secure seal and a decorative edge, use the tines of a fork to crimp the edges of each pastry.
6. Apply the Egg Wash: Arrange the sealed pastries on your prepared baking sheet, leaving about an inch of space between each one to allow for puffing. Add a teaspoon of water or milk to the reserved whisked egg and mix to create the egg wash. Using a pastry brush, lightly brush the tops of each pastry with the egg wash. This is the secret to that deep, golden-brown, glossy finish.
7. Final Touches: If desired, sprinkle the tops of the pastries with sesame seeds or poppy seeds. Using the tip of a sharp knife, cut a small slit in the top of each pastry. This acts as a vent for steam to escape, which helps the pastry puff up evenly and prevents the bottoms from becoming soggy.
8. Bake to Perfection: Place the baking sheet in the preheated oven. Bake for 20-25 minutes, or until the pastries are deeply golden-brown, puffed up, and flaky. The exact time can vary depending on your oven, so keep an eye on them during the last few minutes.
9. Cool and Serve: 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. They are best served warm, but are also delicious at room temperature.