Roast the turkey pieces (you should have 5–6 pounds/2–3 kilos) in a hot oven, 425°F/218°C at least, till it looks delicious. Scatter the onion, carrot, and celery in the same pan, and take them out when you take out the turkey. Don't let the veg burn. (You can cut the meat off the bones for dinner if you wish; but the meat will add lots of flavor to the stock.)
Put the turkey bones in a big pot and cover them completely with water, 3–4 quarts/liters, and put the pot over high heat. Turn your oven to 180°–200°F/80°–90°C. When the water comes to a simmer, put the pot in the oven for 8 hours or overnight.
Add the remaining ingredients (if you don’t have enough room, remove the turkey bones—they will have cooked out by now). Bring to a simmer, then reduce the temperature to low and cook for another hour or so. Strain into a clean pot. Cool, then refrigerate.
Reserve any fat that’s congealed on top for the roux on Thanksgiving day. Reduce the stock to 1 ½ to 2 quarts/liters if it's not already at that level.