Special equipment: a large brining bag, optional and a probe thermometer
For the maple-whiskey brine: Combine the whiskey, maple syrup, salt, peppercorns, bay leaves, garlic, rosemary, orange peel, apple peel and 8 cups (2 quarts) water in a large pot. Bring to a boil uncovered, stirring to dissolve the salt, then turn off the heat and cover. Allow to cool completely, then place in the fridge to chill.
For the turkey: Remove the turkey from its packaging and remove the bags inside that hold the neck and giblets. Rinse the neck and giblets and put them in a plastic bag in the fridge; you'll need them for the Maple-Whiskey Gravy. Rinse the turkey thoroughly with cold water. Place the turkey in a large brining bag or pot. Pour in the maple-whiskey brine, cover with cold water and place in the fridge for 16 to 24 hours so the brine can work its magic.
When ready for roasting, preheat the oven to 275 degrees F.
Remove the turkey from the brine and rinse it thoroughly again inside and out. Soak the turkey in cold water for 15 to 20 minutes, then rinse again and pat dry. Tuck the wings underneath the turkey, then tie the legs together with kitchen string. Place the turkey breast-side up on a rack in a large, shallow roasting pan. Cover the pan with heavy-duty foil so that it's well sealed. Roast for about 10 minutes per pound (about 2 1/2 hours for a 15-pound turkey and about 3 hours 20 minutes for a 20-pound turkey).
Remove the turkey from the oven and increase the oven temperature to 375 degrees F. Remove the foil and set aside. (Put stuffing in the bird if you wish at this point.) Rub the butter all over the skin of the turkey, getting in the crevices. Insert a probe thermometer into the thigh, near the hip joint. Place the turkey, uncovered, back in the oven. Continue roasting, basting or brushing with the juices in the pan every 30 minutes, until the thermometer registers 165 degrees F and the juices are no longer pink. This could take another 2 to 2 1/2 hours, depending on the size of the bird.
Remove the turkey from the oven and cover lightly with foil until you are ready to carve it. Pour the pan drippings into a fat separator and set aside for the Maple-Whiskey Gravy.
Put the giblets and neck in a small saucepan of water over medium heat and bring to a boil. Turn the heat to medium low and simmer until the giblets are cooked, 45 minutes to 1 hour. Remove the giblets and neck, set aside and keep the water in the saucepan.
In the turkey roasting pan (which should not have been cleaned!), add back 1/4 cup turkey fat. Sprinkle in the flour and whisk it into the fat to form a paste. If the paste is too thick or clumpy, add a little more fat. If it's too greasy, sprinkle in more flour. Whisk and cook the roux over medium-low heat until deep golden brown.
Turn off the heat and add the whiskey, then turn the heat back on and whisk it in. Pour in the chicken broth, whisking the whole time, then add the maple syrup. Allow it to cook and thicken for several minutes, whisking occasionally. If the gravy gets too thick or if it's too salty, thin it with a little of the giblet water. If the gravy is too thin, just keep cooking until it thickens up. Taste, then add salt and plenty of black pepper. Chop the giblets and add them to the gravy. Remove as much of the neck meat as you can and add it to the gravy too. Serve immediately!