Preheat oven to 425 degrees F.
In a large saucepan, combine cubed chicken, diced carrots and celery, frozen peas and 1 bouillon cube. Cover with water and heat until boiling. Let boil for 15 minutes and then drain.
While first pot boils, In another large saucepan cook onions in the butter over medium heat until translucent (about 5 minutes). Stir in flour, salt, pepper and garlic powder. Then slowly stir in chicken broth and milk. Simmer over medium low heat until thick, then remove from heat.
Place one of the pie crusts in the bottom of a pie pan and top with the cooked chicken, vegetable mixture (once it's drained). Pour the hot onion liquid mixture over the chicken and vegetables in the pie crust. Cover with the top crust. Seal edges and cut away excess dough. Make a few small slits in the top of the pie crust to allow steam to escape.
Bake in preheated oven for 30-35 minutes, to until crust is golden brown. Cool for 10 minutes before serving.