Place puff pastry sheets next to each other and allow to thaw. Wash and sort the spinach. Peel and finely dice onions and garlic. Heat oil in a pan and fry both in it.
Add the spinach and stew for 3-4 minutes. Season with salt and pepper. Cut the feta cheese into cubes. Roll out each slice of puff pastry on a floured work surface, each 24 cm long, and cut in half crosswise.
Put some spinach and feta cheese in the middle of each. Separate the egg. Brush the edges of the dough with egg white, fold over and press the cut surfaces together with a fork. Brush everything with egg yolk and place on a baking tray lined with baking paper.
Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for about 15 minutes until golden brown. Serve garnished with parsley. Makes 12 pockets.