Preheat your oven to 375°F (190°C).
In a large cast iron skillet, melt the butter over medium heat. Add the onion and celery, and cook until they become tender, about 5 minutes.
Sprinkle the flour over the onion and celery, stirring constantly for about 1 minute to cook out the raw flour taste.
Slowly whisk in the chicken broth and milk. Continue whisking until the mixture becomes thick and bubbly.
Stir in the shredded chicken, frozen mixed vegetables, salt, pepper, thyme, and garlic powder. Cook for another 5 minutes, until everything is heated through.
Remove the skillet from heat and place one pie crust on top of the chicken mixture, pressing it against the edges of the skillet.
Cut slits into the top crust to allow steam to escape during baking. Optionally, you can brush the crust with an egg wash for a golden finish.
Place the skillet in the preheated oven and bake for 30-35 minutes, or until the crust is golden brown and the filling is bubbling.
Remove the pot pie from the oven and let it cool for a few minutes before serving.