Preheat oven to 180 C.

In a medium sized pot, heat oil and add chicken pieces, garlic powder (or minced garlic) and salt and pepper. Cook on high heat until the chicken has changed color, and is cooked through. Remove from the pot and set aside.

To the same pot, add butter, let it melt and then add the onion and cook until slightly softened and translucent. Now add the carrots, peas and mushrooms and cook for 5-7 minutes until tender.

Now add the flour, and mix. Don't worry if it looks too dry, we'll be adding milk soon, Cook for 1-2 minutes so that the flour is cooked, and then slowly add in the milk, while stirring continuously.

Keep stirring for 5-7 minutes on medium high heat until the sauce has thickened, and then add the chicken. Cook for 1-2 minutes and then transfer to a baking dish.

Cut the puff pastry according to the size of your baking dish, and then gently place it on the top of the dish, to cover it completely. Make sure the edges of the puff pastry seal the sides of the dish. Make a little cross in the middle of the puff pastry (to release steam during baking) and then brush with the egg wash.

Bake for about 20-25 minutes until the puff pastry is golden brown.