Preheat the oven to 175°C or 347°F
In a small saucepan or frying pan heat up the oil and fry the onions until they begin to soften.
Add the (defrosted/thawed) spinach to the onions and cook for another 2 minutes.
Place the spinach in a colander and drain the liquid, by squeezing it out with a spoon. If still too wet, place the mixture in a kitchen towel and squeeze. This is an important step, as you do not want your pastry to end up getting soggy.
In a separate bowl, whisk one egg until the yolk and egg white are combined. Crumble in the feta. Now add the onion-spinach mixture, season with black pepper and stir until all of the ingredients are completely combined.
Unroll the puff pastry. With a knife divide the pastry into approx. 10 cm squares.
Fill each square with approx. 1 tablespoon of spinach feta mixture. Now fold over the sides so you create a triangle shape. With a fork press the edges of the triangle. This give leaves a nice pattern but also ensures the parcel will not come undone while baking.
Whisk the other egg in a separate bowl and use a brush to coat the triangles with the egg mixture.
Place the spinach and feta parcels on a lined baking tray and bake for approx. 20 minutes until golden.
Leave to cool before serving. Serve at room temperature.