Preheat your oven to 425°F (220°C).
In a large skillet, sauté the chopped onion over medium heat until translucent, about 5 minutes.
Sprinkle the flour over the onions and stir for 1-2 minutes to cook out the raw flavor.
Slowly whisk in the chicken broth and milk, stirring constantly until the mixture thickens, about 3-5 minutes.
Add the shredded chicken, frozen mixed vegetables, garlic powder, thyme, salt, and pepper to the sauce, stirring to combine.
Roll out one pie crust and fit it into the bottom of a greased 9x13 inch baking dish.
Pour the chicken and vegetable mixture over the crust, spreading it evenly.
Top with the second pie crust, sealing the edges with a fork and cutting slits in the top for steam to escape.
Bake in the preheated oven for 30-35 minutes, or until the crust is golden brown.
Let the casserole cool for 10 minutes before serving.