Preheat oven to 425°. In a large pot, combine chicken, carrots, peas, celery, & potatoes. Add water to cover and boil for 15 min.
Remove from heat, drain, and set aside.
In saucepan, over medium heat, cook onions in butter until soft and translucent. Stir in flour, salt, pepper, and celery seed. (Making roux). Slowly stir in chicken broth and milk, making sure all ingredients are smoothly combined. Simmer over medium-low heat until thick. Remove from heat & set aside.
Place one crust in bottom of deep pie dish. Spoon chicken mixture in pie dish. Pour hot liquid mixture over this. Cover with top crust, seal edges, and crimp edges. Make small slits in crust to allow steam to vent.
Bake in preheated over for 30 min., covering edge of crust with foil after 10 min. Crust should be golden brown.