Preheat oven to 400°F.
Pat dry turkey wings and arrange in roasting dish with onion segments. Roast for about an hour or until well browned (you may need an additional 15-20 minutes).
Transfer wings and onion quarters to a large pot. Use 1 cup water in the bottom of the roasting pan to help scrape up any brown bits and add them to the pot as well.
Add carrots and thyme to the pot with the turkey wings and fill with 6 cups of the chicken stock. Use additional water if needed to ensure the turkey wings are mostly submerged (it's okay if a few tips are sticking out above the water).
Bring to a vigorous boil, then reduce to a simmer. Let simmer, uncovered, for 1½ hours.
Remove the wings and set aside — once they're cool enough to handle, shred the meat off to eat.
Strain the liquid into a smaller pot to remove the carrots and herbs, squeezing as much liquid out of them as possible. Those can be discarded. Return the pot of broth to the stove, skim any fat off the top, and bring back to a boil.
In a separate bowl or large measuring cup combine the remaining 2 cups chicken broth with the ¾ cups all purpose flour. Whisk well to combine and break up any lumps (you can sift the flour first to be really sure you won't get lumps, but you don't have to).
Whisk in the chicken broth and flour mixture into the turkey stock. Use a small wire strainer/skimmer/spider or spoon to remove any lumps of flour that float to the top. Bring to a boil and keep whisking for 3-4 minutes until quite thick.
Stir in butter, pepper, and salt. Dip shredded turkey meat into the gravy to taste and adjust salt and pepper as needed.
Make ahead turkey gravy will stay good in the fridge for 1 week, or in the freezer up to 3 months. When reheating from fridge or freezer make sure to whisk well before serving.