Preheat your oven to 425°F (220°C).

1. In a large skillet or saucepan, melt the butter over medium heat. Add the diced onions, celery, and carrots, and cook for 5-7 minutes, or until the vegetables are softened.
2. Stir in the garlic powder, thyme, salt, and pepper. Sprinkle the flour over the vegetables and cook for 1-2 minutes, stirring constantly, to create a roux (paste).
3. Gradually add the chicken broth and milk, stirring constantly until the mixture thickens and becomes smooth, about 5 minutes.
4. Add the cooked chicken and frozen peas to the skillet, and mix everything together. Let the mixture simmer for another 5 minutes until heated through. Taste and adjust the seasoning if needed.
5. Roll out one of the pie crusts and fit it into the bottom of a 9-inch pie pan or baking dish. Press gently to ensure the crust is snug against the edges.
6. Pour the chicken mixture into the prepared crust and spread it out evenly.
7. Roll out the second pie crust and place it on top of the filled pie. Trim any excess crust, then crimp the edges to seal the pie. Cut a few small slits in the top crust to allow steam to escape while baking.
8. Place the pie on a baking sheet to catch any drips, and bake in the preheated oven for 30-35 minutes, or until the crust is golden brown and the filling is bubbling.
9. If the crust begins to brown too quickly, cover the edges with aluminum foil to prevent burning.
10. Let the Chicken Pot Pie cool for 5-10 minutes before slicing and serving. This allows the filling to set slightly and makes it easier to cut.