Preheat oven to 425 degrees F. Spray a 9 inch pie dish (preferably glass) with cooking spray and gently press one of the crust into the bottom pushing the sides down evenly. Bake in the preheated oven for about 8 minutes.
Meanwhile, combine carrots, peas, and celery in a saucepan and cover with water. Bring to a boil and boil for 15 minutes. Drain and stir the chicken into the vegetable mix.
Cook the onions with melted butter over medium heat until soft. Stir in flour slowly (use a sifter if you have one) stirring continuously, add pepper, salt, and garlic powder. Slowly stir in chicken broth and milk. Simmer over medium low heat until thick. Remove from heat.
Spread the chicken mixture evenly in the bottom pie crust. Pour hot broth mixture over the chicken up to ¼ inch from the top of the pie dish and cover with the top crust, pinching edges into the rim of the pie plate/bottom crust.
Bake in the preheated oven on a cookie sheet for 30-35 minutes or until pie is golden and filling is bubbly.
Cool for ten minutes before cutting.
