Place chicken breasts into a pot. Fill with just enough water to cover, then add garlic powder, salt, and pepper. Bring to a boil, then remove from the heat, cover, and let cool in the pot.

Meanwhile, place potatoes into a large pot and cover with salted water; bring to a boil. Reduce the heat to medium-low and simmer until almost fork-tender, about 15 minutes. Drain and set aside.

Preheat oven to 350 degrees F (175 degrees C).

Heat oil in a large heavy skillet over medium heat; add onion and celery and sauté for 5 to 8 minutes. Add frozen vegetables and cook for 5 minutes. Stir in flour and cook for about 30 seconds. Add chicken broth and bring to a boil. Cook until broth has thickened, then add potatoes.

Remove cooled chicken from the pot. Discard bones and cut meat into bite-sized pieces. Add meat to vegetable mixture and season with salt and pepper.

Press one pie pastry into a pie pan. Pour in chicken and vegetable mixture, then cover with remaining pie pastry. Crimp pie pastry edges together so no juices spill in the oven.