Preheat oven to 350°F. Line a baking tray with parchment paper.
Remove pie shell from freezer if frozen. If refrigerated, add bottom crust to pie tin.
Defrost frozen vegetables by running under hot water and drain.
In a large skillet on medium-low heat, add butter, celery, and onion and cook for 4 minutes, or until onion is translucent (but not browned).
Add flour, poultry seasoning if using, and whisk (or stir) while cooking for one minute. Turn to medium (or medium high) and slowly add chicken broth while whisking (or stirring). Cook for about 4 minutes, or until sauce has thickened. Remove from heat and stir in defrosted frozen vegetables, chicken, salt, and pepper.
Add chicken mixture to the crust in the pie tin, and top with second crust. Gently place the edge of the top crust behind the bottom crust. Press edges together to seal. Add slits to the top of the crust to allow steam to escape.
Bake for 1 hour, or until golden brown and bubbling*.
Remove from the oven and allow to rest for 10 minutes before serving.
Notes
*If you find the crust getting too browned, cover the edges with a pie crust shield (or foil), or tent pie with foil for remaining baking time.
Refrigerate leftover chicken pot pie up to 4 days.
Reheat portions covered with foil in the oven, or simply heat in the microwave.
Freeze portions in labeled containers for up to 4 weeks.