Have your chicken cooked and cubed or shredded-set aside. Preheat your oven to 350 degrees.
Heat a large skillet over med-low heat and add the 6 tablespoon butter. Add the onions, garlic and celery, sprinkle with just a little salt and pepper. Cook about 5 minutes until fragrant.
Whisk in the cornstarch and continue whisking 1-2 minutes until thickened.
Add broth then raise the heat, bringing the mixture to a boil while stirring. Once boiling, lower heat to med-low and continue simmering until thickened.
Once thickened, add the milk, poultry seasoning, salt, pepper, chicken, carrots and broccoli. Stir and cook another 1-2 minutes, then remove from heat.
Pre-bake the bottom crust according to package directions, only for 10-12 minutes, or until bottom is just set, then remove from oven.
Fill your pie crust with filling up to the top. If you have extra filling, it can be saved as leftovers for soup, or frozen another use.
Add the top pie shell. If defrosted, you can roll it out in between parchment paper to shape as needed, or cut it in strips like I did.
If using the egg wash, gently brush it over the top crusts and sprinkle with additional salt if desired.
Place pie dishes on a large baking sheet just in case they boil over while baking! Bake in the preheated oven for about 30 minutes, or until crust is deep golden brown and filling is bubbling. Cover pie crust edges with foil to avoid burning. Enjoy!