Once the stock is boiling, add about half of the flour-butter-dripping mix to it, whisking the stock vigorously all the while. Let it boil for a couple minutes, then knock back the heat and simmer for about 5 minutes. Check the thickness—if you want it thicker, repeat the boil-whisk-simmer process above. The additional thickening required will vary; the natural collagen and gelatin in the turkey bones and skin help to aid thickening (which is why the stock will shake like Jell-O when it’s cold), but the levels of collagen vary slightly per animal, as do our preferences for gravy thickness. I usually end up using the whole flour mix in mine.