1. Preheat oven to 425 degrees F.
2. In a saucepan, combine carrots, peas and celery. Add enough water to cover vegetables and boil for 15 minutes. Remove from heat, drain, stir in chicken and set aside.
3. Melt butter in a saucepan and stir in flour, salt, pepper, onion and garlic powder. Slowly stir in chicken broth and milk. Simmer over medium-low heat until thick. Remove from heat and set aside.
4. Form one pie crust into the bottom of a pie pan or dish and place the chicken/veggie mixture on top of it. Pour hot liquid mixture over, being careful not to overfill. Cover with top crust, seal edges and cut away excess dough. Make several small slits in the top to allow steam to escape.
5. Place on a baking sheet in case of spills and bake in the preheated oven for 30 to 35 minutes, or until pastry is golden and filling is bubbly. Cool for 10 minutes before serving.