Wash chicken breasts in water and then place in a pot with water just to cover chicken; add some salt and pepper and garlic powder. Bring to boil, turn off heat and cover. Let cool in the pot. Preheat oven to 350° . Wash and cut potatoes into bite size pieces and boil till almost fork tender. Drain and set aside. Wash celery and cut to bite-size pieces. Cut onion to bite-size pieces. In large heavy skillet, over medium heat, sauté celery and onion with 2 tablespoons oil, for 5 to 8 minutes. Add frozen vegetables and cook another 5 minutes. Add flour and cook about 30 seconds.
Add chicken broth and bring to a boil. After it has become thick, add potatoes.
Remove chicken from pot and cut into bite-size pieces and add to vegetable mixture. Add salt and pepper to taste. Pour mixture into a 9 inch deep dish pie plate and cover with pie shell. Make sure you crimp the edges of the pie shell to the plate so no juices spill in oven. Bake at 350° for 45 minutes or until pie shell is cooked and golden brown.