Line a baking tray with parchment paper. Set aside.
On a floured surface open the pie dough into a large circle of about 5mm thickness. From the circle cut equal sized rectangles. The recommended size of each rectangle should be 15cm x 9cm.
Fill half of the rectangle with the leek and mushroom mixture and sprinkle the crumbled feta cheese on top. Make sure to leave space around the edges of the filling.
With a pastry brush, paint the edges of the dough around the filling with the egg wash. This will help you achieve a secure closing of the pies. Fold over the other half of the dough and press the ends together around the filling with your finger. You can either leave them as such or else press them down further with a fork. On the top part of the pies, slit two small cuts with a sharp knife.
Place the pies on the lined baking tray and place in the fridge for about 30 minutes. This step helps the dough to remain closed while baking, although no worries if some of the juices ooze out while baking.
Preheat the oven to 220°C or 430°F.**
Once ready to bake, brush the top and sides of the pies with the egg wash, and sprinkle with some seeds of your choice. I sprinkled some of mine with nigella seeds and some others with za'atar.
Bake the pies for about 20 minutes or until they have a nice golden brown colour.
Let the pies cool slightly before serving and enjoy!