Preheat the oven to 425°F (220°C).

In a large skillet, melt the butter over medium heat, then add the diced onion and diced celery and cook until softened, about 5 minutes.

Stir in the all-purpose flour and cook for 1-2 minutes until golden to form a roux.

Gradually whisk in the chicken broth and milk, stirring until smooth and thickened, about 3-5 minutes.

Add the salt, black pepper, garlic powder, thyme, shredded chicken, and frozen peas and carrots and stir to combine.

Roll out one refrigerated pie crust and place it in a 9-inch oven-safe baking dish.

Pour the chicken filling into the crust, cover with the second pie crust, and seal the edges by crimping them together.

Cut slits in the top crust to allow steam to escape and bake at 425°F (220°C) for 30-35 minutes until the crust is golden brown.

Let the pie rest for 10 minutes before serving to allow the filling to set.