01 - Preheat the oven to 400°F (200°C).
02 - Melt butter in a large skillet over medium heat. Add onion, carrots, and celery; sauté until softened, approximately 5 minutes.
03 - Stir in all-purpose flour and cook while stirring constantly for 1 minute.
04 - Gradually whisk in chicken broth and milk. Continue cooking and stirring until the sauce thickens, about 3 to 5 minutes.
05 - Add cooked chicken, frozen peas, salt, black pepper, dried thyme, and garlic powder. Mix thoroughly and remove from heat.
06 - Place one pie crust into a 9-inch pie dish. Pour the prepared filling evenly into the crust.
07 - Cover filling with the second pie crust. Trim excess dough, then crimp edges to seal. Cut small slits on top crust to allow steam to escape.
08 - Brush the top crust with beaten egg to achieve a golden color during baking.
09 - Bake for 35 to 40 minutes until the crust is golden brown and the filling is bubbling.
10 - Allow the pot pie to cool for 10 minutes prior to serving.