In a large saucepan, melt the butter over medium heat. Add the chopped onion and celery, cooking until softened (about 3-4 minutes).
Stir in the flour and cook for an additional minute to create a roux.
Gradually whisk in the chicken broth and milk, ensuring there are no lumps. Cook until the mixture thickens, about 5-7 minutes.
Add the cooked chicken, peas and carrots, cooked potatoes, thyme, garlic powder, salt, and pepper to the saucepan. Mix well until everything is combined and heated through.
Transfer the chicken mixture into a baking dish.
Roll out your pie crust and carefully lay it over the filling, pressing down the edges to seal. Cut a few slits in the top for steam to escape.
Brush the top of the pie crust with the beaten egg for a golden finish.
Bake in the preheated oven for 30-35 minutes, or until the crust is golden brown and the filling is bubbling.
Allow to cool for a few minutes before serving.
Notes
Serve slices on rustic plates and garnish with freshly chopped parsley for a pop of color. Enjoy with a side of fresh salad for a wholesome meal!