Season the chicken thighs all over with 2 teaspoons of salt, 1 teaspoon of pepper, and the poultry seasoning.
In a large Dutch oven, heat the vegetable oil over medium heat. Place seasoned chicken, skin-side down, in the hot oil and begin to render the chicken fat, about 5 minutes. Once the skin in crispy, flip the chicken and continue cooking for 3 minutes more. Remove the chicken from pot and set aside. Pour the chicken fat into a small bowl and reserve.
Add the water, onion, the chopped carrot, chopped celery, and parsley stems to the pot. Return the chicken thighs to the pot and bring to a boil over medium high heat. Once boiling, reduce the heat to low and simmer for 30 minutes, or until the chicken is cooked through.
Remove the chicken from the pot and discard the skin and bones. Shred the meat and set aside. Strain the stock through a fine-mesh sieve and discard the vegetables.
Add 1 tablespoon of the reserved chicken fat to the pot over medium heat, along with the sliced carrots, sliced celery, garlic, and 2 tablespoons of flour. Stir to combine.
Slowly add the reserved chicken stock, 2 teaspoons of salt, 1 teaspoon of pepper, and the garlic powder. Add the shredded chicken back to the pot and bring to a simmer over low heat and continue cooking for 20 minutes or until the vegetables are tender.
Meanwhile, make the dumplings: In a medium bowl, combine the remaining cups of flour, the baking powder, remaining teaspoon of salt, remaining ½ teaspoon of pepper, and the remaining 3 tablespoons of reserved chicken fat. Mix with a fork to combine. Add the buttermilk and mix to incorporate, then knead for 2 minutes, until the dough comes together.
Pull off about 1 tablespoon of dough at a time and add to the soup. Continue cooking until the dumplings are cooked through, about 10 minutes.
Ladle into bowls and garnish with the parsley leaves.