1 day to 1 hour before roasting: Line a large rimmed sheet pan or roasting dish (one that is large enough to hold the turkey) with foil. If using a sheet pan, place an oven-safe cooling rack on top.
Spatchcock turkey: Place turkey, breast side down on a large cutting board. Pat it dry with paper towels (this makes it easier to work with). Use sharp kitchen shears to cut along both sides of the backbone, completely removing it. (Save the backbone to use for making stock.) Open up the turkey and use the tip of a knife to make small slashes / score along one side of the breast bone (this makes it easier to flatten).
Turn turkey so the breast side is facing up. Using the palms of your hand, press firmly down on the turkey breast until the bones crack and the turkey rests flat. Transfer spatchcocked turkey to prepared sheet pan.
Pat turkey dry with paper towels. Tuck wing tips under the turkey (so they don’t burn when roasting).
Season: Gently loosen the skin by running your fingers (or a small spoon) between the skin on the breast and legs. Rub salt under the skin and all over the turkey, concentrating it where the meat is thickest (like the breast). Let turkey come to room temperature (or return it to the refrigerator and let rest, covered with plastic wrap, for up to 24 hours).
Preheat oven to 425°F / 218°C.
Combine softened butter, herbs, and black pepper. Rub seasoned butter under and over the skin of the turkey. Season the surface of the turkey with some additional kosher salt.
Pour stock into pan under turkey. If using chopped onions, celery, carrots, sliced garlic heads, apples, and / or sprigs of fresh herbs to flavor the turkey, lift the wire rack with turkey up and add those to the bottom of the pan.
Carefully transfer turkey to the lower rack of the heated oven and roast for 70 minutes to 2 hours. Start checking turkey at the lower end of the cook time and continue checking until the thickest part of the breast registers 160°F / 71°C and the thickest part of the thigh registers 165°F / 73°C.)
Transfer turkey to a cutting board and let rest for 30 minutes (you can tent it with foil to keep it warm). While turkey rests, use pan drippings to make gravy.
Carve turkey and serve!