1. In a large bowl, place the flour and salt and mix. Add the sunflower oil or vegetable shortening, the egg, and the yogurt, and knead until you have a soft, pliable dough that doesn't stick to your hands. Cover it and let it rest while we prepare the filling.
For the filling:
1. Shred the boiled chicken and cut it into small pieces. You should have 3 cups of cooked chicken.
2. In a frying pan, heat the olive oil and sauté the spring onions and leek until softened.
3. Add the mushrooms and corn and stir. Add the chicken, parsley, salt, pepper, and grated halloumi.
Prepare the béchamel:
1. In a saucepan, put all the ingredients for the béchamel and mix.
2. Place it on the heat and stir until it thickens. Add the béchamel to the chicken mixture and stir.
3. Let it cool down a bit.
For the chicken pie:
1. Grease the bottom and sides of a baking pan well. Divide the dough into two parts. Take one piece of non-stick parchment paper and roll out one piece of dough into a large sheet that covers the bottom and sides of the pan. Prick the dough all over with a fork.
2. Spread the filling evenly. Roll out the second sheet, prick it with a fork, and cover the pie, sealing the edges well. Brush with the beaten egg white.
3. Bake the chicken pie in a preheated oven at 180°C until nicely browned and the dough is cooked through, about 50 minutes. Let it rest for a while before cutting and serve warm.