Preheat the oven to 400°F (200°C).
In a large skillet, melt the butter over medium heat. Add the carrots, celery, and peas, and sauté for 5-7 minutes, until the vegetables are tender.
Stir in the flour and cook for 1-2 minutes to form a roux.
Gradually whisk in the chicken broth and heavy cream. Bring to a simmer and cook for another 3-5 minutes, until the sauce thickens.
Add the shredded chicken, thyme, garlic powder, salt, and pepper. Stir well to combine.
Roll out one pie crust and place it in the bottom of a 9-inch pie dish.
Pour the chicken mixture into the pie dish over the crust.
Place the second pie crust on top, pinching the edges together to seal. Cut a few slits in the top to allow steam to escape.
Bake for 30-35 minutes, until the crust is golden brown. Let cool for 5 minutes before serving.