Preheat your oven to 200°C (390°F) and line a baking sheet with parchment paper. Set aside.
Melt the butter in a skillet over medium heat and add the finely chopped onion. Sauté until the onion turns soft and golden, filling your kitchen with a sweet aroma—about 5 minutes.
Add the thawed and drained spinach to the skillet and cook for another 2-3 minutes, stirring until the spinach is heated through and slightly wilted. Remove from heat and let cool slightly.
In a mixing bowl, combine the spinach mixture with grated Parmesan, a pinch of nutmeg, salt, and pepper. Stir well to create a flavorful filling. Chill briefly if necessary to keep it firm for filling.
Unfold the puff pastry sheets on a lightly floured surface and gently roll them out to smooth seams. Cut each sheet into 8 equal squares, about 10cm (4 inches) per side.
Place a teaspoon of the spinach filling in the center of each pastry square. Fold each square into a triangle or rectangle and pinch the edges together to seal tightly. Use a fork to crimp the edges for a secure seal.
Arrange the filled puffs on your prepared baking sheet, spacing them evenly. Brush the tops with the beaten egg to give them a glossy, golden finish.
Bake in the preheated oven for 15–20 minutes, or until the puffs are golden brown and crispy, and the filling is bubbling at the edges.
Remove from the oven and transfer to a wire rack. Let rest for 5 minutes to allow the filling to set slightly and the pastry to cool enough to handle.
Serve warm or at room temperature, garnished with extra Parmesan if desired. Enjoy these flaky, savory bites that blend nostalgia with a touch of sophistication.
