Preheat the oven to 210° C, air circulation and rack set at the middle.
Take a medium pot and sweat the onions with the butter. Add the flour and cook for a minute or so. Pour the milk and stir well aiming for a very thick sauce. Add the frozen vegetables along with the shredded chicken, feta cheese and season with the salt and pepper. Mix well.
In the meantime, take pie pan, about 20cmdiameter, and lightly brush its base with butter. Place the puff pastry andfill with the chicken mixture. Fold in the end of the puff pastry aiming toseal the pie and have a rustic look. Brush it liberally with the egg wash and cutslits at various places to allow release of steam while cooking and place inthe oven cooking for 45’. Allow to cool before slicing and serving next to agreen salad.