Preheat oven to 425 degrees F.
Add the butter to a nonstick pan over medium heat. Once the butter is melted, add the chicken, carrots, celery, onion, salt, garlic powder, and thyme leaves. Cook for 6 minutes, stirring often.
Add the flour. Stir well, until no dry flour remains. Slowly stir in the milk and broth. Cook until bubbling and thick, stirring often, 3-4 minutes.
Remove from the heat. Stir in the peas and parsley. Let this cool for 10 minutes before filling the pie.
Place one pie crust onto a baking sheet. Spoon the filling into the pie crust. Top with the second pie crust. Cut any excess crust off the top and seal the edges of both pie crusts together with a fork. Cut 3-4 slits in the top crust to allow steam to escape.
Bake for 30 minutes on the bottom rack of the oven.
Cool for 15-20 minutes before slicing and serving.