Make the pie crust first. Combine the flour, sugar and salt together in a large bowl or food processor. As I mentioned above, I used my Ninja Blender with the pulse setting, just like a food processor.
Add cubed butter and then pulse again. Combine until all the butter is incorporated into the flour mixture. This mixture will be very flaky. Place mixture into freezer for 15 minutes to completely chill the butter.
Remove from the freezer and add in buttermilk. Using a spoon and then your hands, combine until dough becomes a ball. If the mixture is too dry, you can add 1-2 tablespoons of cold water here. Divide dough into two and flatten into disks. Wrap in plastic wrap and store in the fridge until ready for later use.
To make the filling, melt the butter over medium high heat. Then, add in celery, carrots, onion, and garlic. Cook until tender, about 10 minutes.
Whisk in the flour, salt, pepper, thyme, parsley, chicken broth, and heavy cream. Whisk together until there are no lumps present. Then simmer over medium low heat for 10 more minutes, or until the sauce has thickened. Stir in the shredded cooked chicken (or turkey) and frozen peas. Remove from heat as you assemble the pie dough.
Preheat oven to 400 degrees. Take pie dough out of the fridge and roll both pieces out on a floured surface using a rolling pin until each dough piece reaches a 12 inch circle and is 1/4 inch thick. Transfer one of the dough pieces into a 9 inch pie plate and remove and excess overhang. I saved this extra dough to make bat cut outs for a "spooky" pot pie, but this is totally optional!
Fill the pie dough with the pie filling and then cover with the other piece of dough you rolled out earlier. Seal the edges of the pie together by crimping it with your fingers.
With a sharp knife, cut a few slits in the top of the dough. Then, using a pastry brush, brush the crust with the beaten egg for the egg wash.
Bake in preheated oven for 45 minutes, or until crust is golden brown. Cool for 10 minutes and then serve and enjoy!
