Combine celery, onion, carrot, potatoes, salt, pepper and stock in a heavy saucepan and bring to a boil; reduce to simmer and cook until vegetables are just tender.
In a small bowl, whisk together the gravy mix, cornstarch and a ladle full of the chicken stock from pot. Add to pot of stock/vegetables and continue simmering until slightly thickened. Gently stir in peas.
Lay one crust in deep dish pie plate, covering bottom with chopped chicken. Pour the stock/vegetable mixture over the top, and cover with second pie crust, sealing edges and cutting a couple of slashes in center. Bake at 375 degrees for 30-45 minutes or until bubbly and browned.