Preheat oven to 425°F (220°C).
In a saucepan over medium heat, cook onions in butter until soft and translucent. Stir in flour, salt, pepper, and garlic powder. Slowly stir in chicken broth and milk. Simmer over medium-low heat until thick. Remove from heat and set aside.
Combine chicken, carrots, peas, and celery in a bowl. Add the creamy sauce and mix well.
Place one pie crust in the bottom of a pie dish. Pour the filling into the crust. Cover with the second crust, seal the edges, and make several small slits in the top to allow steam to escape.
Bake in the preheated oven for 30 to 35 minutes, or until pastry is golden brown and filling is bubbly. Cool for 10 minutes before serving.