In a large skillet, melt 2 tablespoons of butter over medium heat. Add the diced onion and cook for about 3 minutes until softened. Add the frozen peas and carrots, cooking for another 3 minutes.

Add the remaining 1 tablespoon of butter to the skillet. Once melted, sprinkle the flour over the vegetables and whisk for 1-2 minutes to form a roux. Gradually add the chicken broth and milk, stirring constantly until the mixture thickens (about 5 minutes).

Stir in the garlic powder, thyme, salt, and pepper. Add the shredded chicken and mix until well combined. Remove from heat.

Preheat your oven to 400°F (200°C). Roll out the first pie crust on a lightly floured surface. Fit it into a 9-inch pie dish. Pour the chicken filling into the pie dish.

Roll out the second pie crust and place it over the filling. Pinch the edges together to seal and cut a few slits in the top to allow steam to escape.

Bake the pie in the preheated oven for 35-40 minutes, or until the crust is golden brown and the filling is bubbly. Let the pie cool for 5 minutes before serving.