Pastry: Sieve the flour into a bowl. Add the vinegar, salt and olive oil, stir, then, add the water, a little at a time, working the dough with your hands until it comes together and has a smooth texture. Cover and let it rest for 30 minutes minimum. Divide it into 4 equal parts. Dust the work surface with flour and roll each part out to a thin round filo sheet.
Filling: In a large pan sauté the spinach over a medium heat with a little olive oil and a pinch of salt, cook in batches until the stems are soft. Place in a strainer (big enough to fit all of the cooked spinach) over a bowl or in the sink. Place a small plate over the spinach and use to press down well to get rid of excess water.
Finely chop the onion and sauté on a low heat in 40g of olive oil until soft and lightly coloured. Once the spinach has been thoroughly squeezed out roughly chop and add to the onion, season lightly with salt (not too much as feta will add further salt) and black pepper. Cook for a minute to combine flavours. Remove from heat, crumble in the feta (small pieces) and mix to combine and set aside to cool. Once cool beat the egg and combine with the spinach mixture.
To assemble: Cut filo into 5cm strips, add 1 tablespoon of spinach mixture to bottom left corner, fold bottom edge up over spinach to meet right edge (forming a triangle), continue folding over in a triangle form until all filo has been used. Place on a greased baking tray and brush with olive oil. Bake at 220°C for 20 mins until golden brown.