Drain your turkey and remove extra parts. Place it on a cutting board, trim the loose neck skin, and flip so the backbone is facing up. Use sharp kitchen shears to cut along both sides of the backbone to remove it. Remove the legs if you want easier handling.
Pat everything dry with paper towels. Mix 3 tablespoons kosher salt with 1 tablespoon baking powder. Sprinkle over the entire bird, including the underside. Place uncovered in the fridge overnight. This step helps with juicy meat and crispy skin.
Mix 1/2 cup softened butter with 6 minced garlic cloves, 2 teaspoons thyme, 2 teaspoons sage, and 1 tablespoon rosemary. Score under the breastbone and press down on the breast until it cracks slightly—this helps it cook evenly. Loosen the skin and spread the herb butter under and over the skin.
Rough chop 2 carrots, 3 celery stalks, and 1 onion. Add them to the bottom of a large roasting pan. Pour in 2 cups of water. Place a rack on top of the veggies and lay the turkey pieces skin-side up.
Roast at 425°F for 60 to 90 minutes. The breast should reach 150°F and the legs 175°F. Turn the pan halfway through and cover with foil if browning too fast. Remove parts as they finish cooking and let them rest.
Transfer the vegetables and drippings to a bowl and skim off the fat. Lightly blend the veggies with an immersion blender and strain into a saucepan. Bring to a simmer, then stir in 1 cup chicken stock mixed with 2 tablespoons cornstarch. Simmer 2–3 minutes until thickened.