1. Break the egg in a bowl and add the salt, pepper & mustard. Beat well and set aside.
2. In a large bowl place all the onion and cook it in the microwave for about 2 minutes once done keep it aside.
3. Great a baking dish or pie dish.
4. Pre heat the oven at 200 degree C / 400 degree F .
5. Dust a clean work surface with flour and roll out the pastry evenly. Once done line the baking/pie dish with the pastry.
6. Combine cheese and the onion together. Spoon the mixture over the pastry base.
7. Pour the beaten egg mixture over it. Brush the edges of the pastry with egg
8. Roll another sheet of the pastry dough to cover the top. Crimp or Pinch the edges. Brush some egg wash on the top of the pastry to give a nice golden color.
9. If you have any extra dough left then roll out and cut into strips to make a pattern that you like as shown in the video.
10. Bake the pie in the preheated oven for 30 minutes.
11. Cut into portions and serve warm as an appetizer or as a side.
After half an hour you can again brush the pastry with egg wash to give it a beautiful glaze.