1. Place the diced onions, knob celery, and oil in a large pot and cover with lid. Over low heat, sauté until very soft, about 30–45 minutes. Stir gently every five to 10 minutes. Add four cups of water to the pot and blend onions and celery with an immersion blender until fully processed.
2. Add an additional 10 cups of water along with the chicken, turkey, flanken, salt, and pepper to the pot. Bring to a boil and then lower the heat to bring the soup to a simmer. Once the oils start to foam up and float on the top of the soup, dip the edge of a shallow skimming utensil just below the surface of the soup and skim off the foam and fat.
3. Once you’ve finished skimming the soup, allow it to simmer for one and a half hours. Add sliced carrots and cook for an additional one and a half hours. Remove the chicken, turkey, and meat from the soup. Separate them from the bones and skin and shred with two forks. Skim some additional fat off the top of the soup and remove any stray bones.
4. Add the shredded meat and poultry back into the soup and mix well. At this point, you may want to thin the soup out with an additional cup or two of water until desired consistency is reached.
