Preheat your oven to the temperature shown on the packaging of your puff pastry.
Heat olive oil in a pan on the hob and add finely diced onions. Sauté until slightly golden.
To the pan, add oregano and spinach. Cook until the spinach has wilted. Remove the pan from the hob.
Remove the mixture from the pan and use a cheese cloth or colander to remove as much moisture/liquid as possible.
Crack open an egg into a bowl and whisk. Remove enough into another container to brush the puff pastry later. Add the spinach/onion mixture to the whisked egg in the bowl. To this, crumble your feta cheese into this and combine all ingredients.
Roll out your puff pastry and mark up squares, 2 for each pasty, depending on how big or small you want them. One square will be the top and one will be the bottom of the pasty. Once you've marked these out, add mixture to the bottom part of each pasty. Mould the filling into a square as seen in the photo above. Ensure you don't overfill as this will cause the pasty to be soggy.
Place the top square on top of the square with the filling and seal the edges with your fingers. Ensure this is firmly sealed so the filling doesn't leak when baking. Use a fork to press down around the square - to replicate the vertical lines as seen in the photos above.
With the small amount of whisked egg set aside from earlier, brush the top of each pasty with the egg. This will help the pasty achieve a golden crust and seal the pasty.
Place the pasties on a baking tray lined with tin foil and place into the oven for approximately 10-15mins. The time will vary depending on how big your squares are. Bake them for at least 10 mins and return to the oven the check on the pastry to see if this has puffed/risen.
Remove from the oven and allow the spanakopitas to cool for 10 mins then enjoy!