Preheat the oven to 450 F
Clean and dice celery. Cube chicken breast. Place chicken and celery in a pot and cover with water. Bring to a boil. Boil for 10 minutes. Remove from heat; drain and save celery and chicken.
While chicken and celery are cooking, cook peas and carrots according to package directions.
Remove onion skin and dice onion. Melt butter in saucepan and sautee onions until translucent.
Remove onion solids from pan. Melt more butter, if necessary, then add flour and whisk to form a roux. Pour chicken (or turkey) stock into the flour roux, adding half a pint, then the other half pint, and whisking until thick. Add milk, whisk until thickened.
Mix chicken, celery, peas, carrots, and onions together. Add mixture to pie crust. Pour sauce over. Add second crust, inverting and crimping ends. Pierce to release steam.
Bake for 30-45 minutes, checking to make sure crust is browning and not burning. If starting to brown too quickly, cover top crust with foil to allow the rest of the pie to cook without burning.
Remove from oven when cooked and allow 15 minutes to cool before serving.