Line a rimmed baking sheet with aluminum foil for easy clean up, and set an oven rack on top.

Remove and discard the truss that holds turkey legs together. Trim off and discard any excess fat in the neck or body cavity. Remove giblets and neck; discard or save for stock. Pat the turkey dry with paper towels.

In a small bowl, mix the sugar, salt, thyme, sage, and pepper for the dry brine. Place the turkey on the rack of the prepared baking sheet. Rub and pat the dry brine all over the turkey, including inside the cavity. Refrigerate, uncovered, for at least 24 hours and up to 48 hours.

Preheat the oven to 375°F and set an oven rack in the lower third of the oven. Using damp paper towels, brush the dry brine off the turkey's exterior.

Scatter the onion, carrots, and celery in the bottom of a large roasting pan. Place a roasting rack inside the pan and place the turkey on the rack. Tuck the wings underneath the bird. Using your hands, smear the butter all over the turkey.

Roast the turkey until the skin is deep golden and a leave-in or instant-read thermometer registers 165°F in the thickest part of the breast and 175°F in the thigh, 1¾ to 2½ hours. Check the turkey after about 1½ hours; if the skin is browning too quickly, cover it loosely with foil.

Using clean oven mitts, carefully tilt the turkey so any juices from the cavity pour into the roasting pan. Transfer the turkey to a platter or cutting board. Tent with foil and let rest for 20 to 30 minutes. Remove the vegetables from the pan and discard. Reserve the drippings in the pan for the gravy.

While the turkey rests, make the gravy.

Carve the turkey and serve with the gravy on the side.