Heat a large pot to medium heat then add chicken fat (or oil), carrots, celery, and onion. Cook for 5 minutes with lid on. Then add frozen peas, chicken brother and poultry seasoning. Mix well, and cook for an additional 5 minutes with lid on.
Once the veggies are soft sprinkle in 1 tablespoon of cornstarch over veggies, mix well then pour 1-½ cups of whole milk into the pot. Milk well and cook for 5 minutes on medium heat. Stir in cooked shredded rotisserie chicken. Cook for 5 minutes until heated through. Sprinkle 1 tablespoon fresh parsley over the chicken pot pie mixture, stir and turn off heat.
*While pot pie mixture cooks preheat oven and cook puff pastry shells to package instructions.
Once the puff pastry shells are cooked, cut out the middles, stuff each shell with chicken pot pie filling and then top with puff pastry lid. Serve and enjoy.