In a medium or large pot, combine 4 cups of the turkey stock with a few sprigs of fresh sage, thyme, 2 teaspoons of salt (if using my homemade stock with no salt, use less if your stock is salted), and black pepper to taste. Simmer for 15 minutes. Discard the herbs.
Once it’s ready, in a blender, add the remaining 1 cup of cold broth with 2/3 cup flour and the Bell's poultry seasoning and blend until smooth.
Add 2 cups of the boiling turkey stock into the blender to temper and blend again, then pour into the pot with the boiling stock.
Add the pan drippings from the turkey at this point and simmer, whisking on low heat until thickened, about 5 to 10 minutes. Taste for salt and adjust as needed.