Make the dry brine. In a small bowl, mix together the salt, pepper, sugar, garlic powder, onion powder, paprika, and sage. Set aside.
Spatchcock the turkey. First, remove the neck and giblets from the turkey and make sure it’s completely defrosted (if using a frozen turkey). Place the turkey on a large cutting board and pat it very dry with paper towels. Arrange the turkey breast-side down on the cutting board with the legs facing you. Using heavy-duty kitchen shears, cut along one side of the turkey spine, separating it from the ribs. Be sure to cut as close to the spine as you can so that you do not end up discarding any more of the turkey meat than necessary. Repeat on the other side of the spine. Flip the turkey so that the breasts face upward and the legs face outward. Using the palm of your hands, press along the breast bone. You should hear a small crack and the bird should lay flat.
Cover the turkey in the dry brine. Use your fingers to separate some of the skin from the breast and leg meat to create some space. Sprinkle the salt mixture liberally all over the turkey. Rub the salt mixture into the turkey, including on the underside and under the skin so that the bird is well coated. Place the turkey on a baking sheet, breast side up, and transfer to the fridge uncovered for 24 hours, up to 48 hours.
Make the herby mayo. Remove the turkey from the fridge an hour or two before you plan to cook it. Combine the mayo, garlic, lemon zest, herbs, dehydrated onion, salt and pepper in a small food processor and blend until smooth.
Preheat an oven to 400 F.
Prep the turkey for the oven. Add the onions, carrots, celery, herbs, and garlic to the bottom of a roasting pan and pour in the turkey stock. Place the turkey, breast side up, in the roasting pan on top of the veggies. Smear the herby mayo all over the turkey (reserving some for leftover sandwiches, if desired!). Transfer the roasting pan to the oven.
Roast and baste the turkey. Start basting the turkey with the pan juice about 45 minutes to an hour into the cooking, until the internal temperature in the thickest part of the turkey leg reaches 170 F (160 F for the breast), about 90 minutes to 2 hours total cooking time. Remove the turkey from the oven and carefully transfer just the turkey to a large cutting board and loosely tent it with foil. Let the turkey rest for 30 minutes before carving.
You can discard the vegetables in the roasting pan and use the drippings to make my foolproof gravy.