Melt the butter in a large skillet over medium heat. Cook the chopped onion, celery and carrots until soft, about 5 minutes. Stir in flour and cook, stirring constantly, about 3-4 minutes.
Slowly whisk in the chicken broth and milk and bring to a simmer. Continue cooking and stirring occasionally until thickened, about 5 minutes. Remove from the heat.
Add the salt, pepper, garlic powder, cayenne (if using), frozen peas and turkey. Gently fold together with a spatula. Check the seasoning and add more salt and/or pepper as desired. Set aside to cool for about 20-30 minutes.
Preheat oven to 375° F. Place a foil lined, rimmed baking sheet or baking stone in the oven to preheat. *
While the filling is cooling, roll out half the pastry and fit into the bottom of a 9-inch deep dish pie plate. Roll out the top crust and have it ready to use as soon as the filling is added.
Pour the cooled turkey mixture into the prepared crust. Quickly top with the second pastry. Trim the excess crust and fold under sealing the pastry with the bottom crust. Crimp the edges in a decorative pattern. Cut 4 or 5 slits in the top crust to vent. Brush the entire top crust with the egg wash. Sprinkle with sesame seeds.
Place the pot pie on the preheated baking sheet. Bake for 45 to 50 minutes or until the filling is bubbly and the crust is golden.