01 - Preheat the oven to 400°F.
02 - In a large skillet over medium heat, melt unsalted butter. Add chopped onion, diced carrots, and diced celery. Sauté for 5 to 6 minutes until vegetables begin to soften.
03 - Stir in all-purpose flour and cook for 1 minute, stirring constantly to form a roux.
04 - Gradually whisk in chicken broth, then buttermilk. Simmer, stirring regularly, until the mixture thickens, approximately 3 minutes.
05 - Blend in sour cream, dried ranch seasoning, garlic powder, black pepper, and salt until well combined.
06 - Fold in diced chicken breast, frozen peas, and freshly chopped parsley. Remove skillet from heat.
07 - Transfer the chicken and vegetable filling into a 9-inch pie dish or suitable baking dish, spreading evenly.
08 - Drape the refrigerated pie dough over the filling, trim any excess dough, and crimp the edges as desired.
09 - Cut a few small slits in the top of the crust to allow steam to escape during baking.
10 - Gently brush the surface of the crust with the beaten egg for a glossy finish.
11 - Bake in the preheated oven for 30 to 35 minutes, or until the crust is golden brown and the filling is bubbling.
12 - Allow the pot pie to rest for 10 minutes before slicing and serving.