Preheat oven to 375°
Heat oil in a large skillet over medium heat. Add the chicken and cook 2-3 minutes, until heated through.
Stir in carrots and celery. Sprinkle with garlic powder, poultry seasoning, and 1/2 tsp. black pepper. Continue cooking and stirring 3-5 minutes, until vegetables are slightly tender.
Add peas and heat through, 1-2 minutes.
Remove from heat and pour into a medium bowl.
In the same skillet used for the vegetables, heat butter over medium heat. Add onions and sauté until softened and fragrant. Stir in flour, salt, 1/2 tsp. black pepper, celery seed.
Pour in chicken broth and milk gradually, stirring after each addition.
Allow mixture to simmer, stirring occasionally, 3-5 minutes until thickened.
Remove pan from heat and fold in all of the chicken and vegetable mixture.
Pour into pie crust. Cover with second crust, cutting a few small slits in the top crust.
Bake 45 minutes, until crust is golden brown. Allow to cool 10 minutes before serving.