In a skillet over medium heat, add the olive oil. Once hot, add the sliced onions and a pinch of salt. Cook them slowly, occasionally stirring, until they are caramelized and golden brown (about 15-20 minutes).

Reduce the heat to low; add the chopped spinach to the skillet. Cook for an additional 2-3 minutes until wilted. Season with salt and pepper to taste, then remove from heat.

In a mixing bowl, combine the caramelized onions, spinach, and shredded cheddar cheese. Mix well to combine all the flavors evenly.

Preheat your oven to 400°F (200°C). Roll out the flaky pastry on a floured surface. Cut it into rectangles, approximately 4x6 inches.

Place a generous tablespoon of the filling mixture onto one half of each pastry rectangle, leaving space around the edges. Fold over the pastry and pinch the edges to seal tightly.

In a small bowl, whisk the egg until combined. Brush the top of each pastry with the egg wash to give them a beautiful golden color when baked.

Place the pastries on a baking sheet lined with parchment paper and bake in the preheated oven for 15-20 minutes, or until golden brown and flaky.

Remove from the oven, let cool for a few minutes, and serve warm. Enjoy!