Cooking turkey in a Dutch oven is truly the quickest, most delicious way I found to cook a whole turkey. Roasting it in the oven is fine; if you brine and butter the bird well, it will be a delight.
In the refrigerator, it will take between 2 and 3 days for a turkey of this size. I always dry brine it when it is partially thawed, after 1 ½ day in the fridge.
Pat the turkey dry inside and outside with kitchen paper. There is no need to wash it.
Mix rub: In a small bowl, mix together the salt, thyme, rosemary, paprika, and pepper.
Combine the butter, chopped sage, nutmeg, garlic powder, salt, and pepper. Set 2 heaped teaspoons of the mixture aside. Smear inside the cavity of the bird with 1 heaped teaspoon of the butter.
Smear turkey: Carefully loosen the breast skin by sliding your fingers under it and gently lifting it. Thoroughly spread the remaining butter under the skin.
Use the butter you set aside to brush the bird.
Stuff with the quartered onions, apples, and sage leaves. Bind the legs with kitchen twine. Place the bird into the Dutch oven.
Heat the stock in a saucepan. Carefully pour the liquid around the turkey, but not on top.
Roast: Cover with the lid and place the pot in the oven. Roast for 1 hour.
Rest for 30 minutes.
Thicken gravy: In a small bowl, mix cornstarch with some cold water. Pour the slurry into the stock while whisking. Let bubble for 1 minute until thickened.