In a saucepan, melt butter over medium heat. Stir in flour, salt, pepper, onion powder, and garlic powder until well blended.
Gradually stir in chicken broth and milk, cooking and stirring until the mixture begins to thicken and bubble.
Stir in chicken, peas, carrots, and celery. Remove from heat.
Place one pie crust in the bottom of a pie dish. Pour the chicken mixture into the crust. Cover with the second pie crust, sealing the edges and cutting away any excess dough. Make several small slits in the top crust to allow steam to escape.
Bake in the preheated oven for 30-35 minutes, or until the crust is golden brown. Let cool for 10 minutes before serving.