- Preheat and prepare: Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
- Prepare the spinach: If using fresh spinach, cook it in a skillet over medium heat for 2–3 minutes until wilted. Transfer to a clean kitchen towel or fine-mesh strainer and squeeze out as much excess moisture as possible. Roughly chop.
- Make the filling: In a medium bowl, combine the cooked spinach, ricotta, mozzarella, and 1 of the eggs. Stir until well blended. Season with salt and pepper. Set aside.
- Assemble the squares: On a lightly floured surface, gently roll out each puff pastry sheet to smooth any creases. Cut each sheet into 4 equal squares (or 9 for smaller appetizers). Place half of the squares on the prepared baking sheet.
- Fill: Spoon a portion of the spinach mixture onto the center of each square on the baking sheet, leaving a small border around the edges.
- Top and seal: Brush the edges of the filled squares with a little water. Place the remaining puff pastry squares on top. Use a fork to press the edges together firmly to seal.
- Egg wash: In a small bowl, beat the remaining egg. Brush the top of each pastry square with the egg wash. Sprinkle with sesame seeds or poppy seeds if desired. Cut a small slit in the top of each square to allow steam to escape.
- Bake: Bake for 15–20 minutes, or until the pastry is puffed, deeply golden, and crisp.
- Cool and serve: Let cool on the baking sheet for 5 minutes before serving. These are delicious warm or at room temperature.
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