Place chicken in a stockpot and add just enough water to cover; stir in poultry seasoning, sage, garlic powder, salt, and black pepper. Bring chicken mixture to a boil, reduce heat to medium and simmer until chicken is no longer pink at the bone and the juices run clear, 30 to 60. An instant-read thermometer inserted into the thickest part of the thigh, near the bone should read 165 degrees F (74 degrees C).
Remove chicken from cooking liquid; reserve cooking liquid, remove and discard bones and skin from chicken and dice chicken meat.
Preheat oven to 375 degrees F (190 degrees C). Spray a pie plate with cooking spray and line it with 1 of the prepared pie pastries.
Stir diced chicken meat, potatoes, onion, carrots, and mixed vegetables together in a bowl. Pour chicken mixture into prepared pie plate.
Bring 1 cup reserved cooking liquid to a boil in a saucepan. Whisk cornstarch into cooking liquid and cook, whisking constantly, until gravy thickens, 3 to 5 minutes. Season gravy with salt and black pepper. Pour gravy over chicken mixture into prepared pie plate. Spread remaining pie pastry over the top, crimp edges together, and cut a slit in the top of the pie.
Bake in the preheated oven until heated through and crust is golden, about 1 hour. Cool for 5 to 10 minutes before slicing.
