Preheat oven to 425°. Line a baking sheet with parchment paper.
Place puff pastry shells on prepared pan. Bake for 30 minutes. Remove top of each pastry shell.
In a large skillet, melt butter over medium heat. Add onion, carrot, salt, pepper, celery salt, and poultry seasoning; cook until onions are translucent. Whisk in flour until smooth. Cook, whisking frequently, until flour turns a deep golden brown. Slowly add milk, stirring constantly as sauce thickens. Fold in chicken, peas, mushrooms, and pimientos.
Fill each puff pastry with ¾ cup chicken mixture. Garnish with chives, if desired.
Notes
*We used Pepperidge Farm.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/classic-chicken-a-la-king-recipe/