Preheat your oven to 400°F (200°C).
Unroll one Pillsbury refrigerated pie crust and fit it into a 9-inch pie dish.
Melt 1/3 cup butter in a skillet over medium heat. Add 1/2 cup celery, and cook for 2-3 minutes until softened.
Stir in 1/3 cup flour, cooking for 1 minute.
Gradually add 1 1/2 cups chicken broth and 1/2 cup milk, stirring constantly until the mixture thickens and becomes creamy.
Add 2 cups cooked chicken, 1 cup peas, 1 cup carrots, and 1/2 cup corn (optional), and season with garlic powder, onion powder, salt, black pepper, and thyme. Cook for another 2-3 minutes.
Pour the filling into the prepared pie crust and top with the second crust. Seal the edges and cut slits in the top to allow steam to escape.
Bake for 30-35 minutes or until the crust is golden and the filling is bubbling.
Let the pie rest for 5-10 minutes before serving.