Simmer all the broth ingredients together for at least 30 min and up to an hour. Strain and set aside.
Meanwihle, combine the flour, baking powder, salt, pepper, and sage in a mixing bowl. Cut diced butter into the mixture with your fingers or a pastry cutter until the mixture resembles course meal. Gently fold in the buttermilk until just combined taking care to not over mix lest your dumplings be tough. Set aside.
After straining the stock and making your dumpling dough, make the soup. Melt the two tablespoons of butter in a heavy bottom pot and cook the garlic and onion over low heat until fragrant and transluscent, and the onion releases it’s juices about 5-7 minutes, being careful not to brown it. Add the carrot and celery and cook for 10 minutes on medium heat until they start to soften. Pour the broth over the veggies and simmer until the veggies are tender.
Add the shredded turkey and simmer for a minute or two with the additional 1 tablespoon of butter added, stirring to incorporate. While it simmers, mix the two tablespoons of flour with 1/4 cup of water to form a slurry, and stir that into the stew. Cook 10-15 minutes to thicken and cook out the raw flower.
Add the dumplings, a scant tablespoon of dough at time to the soup until the surface is dotted with dumplings, and simmer covered for 10-20 minutes until they are cooked through. They should be soft inside, not wet or gooey. If they aren’t cooked through, cook 2-3 minutes more until done. Serve into bowls and garnish with parsley and celery leaf and fried turkey skin if you have it. I always like an extra crack of pepper on mine!