Heat the butter in a large pot until melted. Add the onions and celery and cook until soft, 2-3 minutes. Stir in the salt, pepper, and poultry seasoning.
Pour 3 cups of the broth into the veggies.
Whisk together the remaining 1 cup of broth with the cornstarch. Stir into the veggie mixture along with the half & half, chicken, and veggies. Bring to a simmer and cook 20 minutes.
Meanwhile, preheat your oven to 425 degrees. Lay your pie crust on a parchment lined baking tray. Sprinkle with salt and bake 10 minutes. Break into pieces and serve with the soup.