Put the chicken, peas, carrots and sweet corn in a pot, then add 2 cups of water sprinkle a bit of salt, quarter tsp. garlic paste and quarter tsp. black pepper. Simmer on medium heat whilst covered for 15 minutes. Drain but reserve the liquid/broth.
Heat the butter, then add the chopped onion and fry until softened. Sprinkle the flour and cook on low heat for 2-3 minutes. Add the black pepper powder. Stir for a few seconds then gradually add the milk and broth whilst stirring continuously to avoid lumps. Simmer until the gravy thickens, then add the boiled veggies and chicken and adjust salt/pepper. Sprinkle the chopped coriander & chopped chillies. Cook together for a minute and then turn off the heat. Add in the lemon juice.
Preheat the oven at 200 C.
Place one of the rolled out pastry sheets in your lightly buttered pie plate. Using a fork, pierce many holes on it to prevent it from puffing up during baking, then bake in a pre-heated oven for about 15 minutes or until lightly golden. Remove the pie plate from the oven and let the bottom pastry cool.
You can skip using the lower crust if you want and just use one pastry to cover the top of the pie. I like to pre-bake the bottom pastry so it becomes nice and crispy before adding the filling.
Now pour the cooked chicken and veggie mixture on the pre-baked crust in the plate. (If you plan on using just one pie crust to cover the top only, then just pour the mixture directly into a greased pie plate). Level everything out then top the mixture with the second pastry, cut out any excess pastry and crimp the edges for a pretty appearance. Using a sharp knife, cut slits into the pie to help the steam escape during baking.
Brush the pie with egg yolk, then bake for about 30-45 minutes or until the pie is golden brown and crispy. Let the pie sit for about 10 minutes before slicing.
Serve with fresh salad and fries if you want.