Make sure your turkey is fully thawed before starting. Generally, allow for 1 day of thawing per 5 pounds of turkey.
Day 1: Brining
Remove the giblets from the turkey cavity. Dry off the turkey all over using paper towels.
Place the turkey on a cutting board with the breast side down. Locate the backbone and cut along one side of it using sharp kitchen shears. Next, cut along the other side of the backbone and then remove it. You can save this bone to make stock or gravy.
Flip the turkey over so the breast side is facing up. Using both hands, press down on the turkey until it rests flat on the board. Transfer to a rimmed sheet pan with a wire rack inserted.
Rub kosher salt all over the turkey. You can optionally rub some of it under the skin. Place the turkey, uncovered, in the refrigerator and let sit overnight, or approximately one hour per pound.
Day 2: Cooking
The next day, let the turkey come to room temperature for one hour. While the turkey is resting, rinse out the sheet pan.
After rinsing out the pan, add the carrot, celery and onion. Top with the wire rack.
Place the softened butter in a bowl. Stir in the chopped herbs, zest, and a pinch of salt until evenly combined.
Pat the turkey dry again, then rub the herb butter all over. Season with fine sea salt and ground black pepper.
Roast the turkey at 400°F until the breast meat reads 150 degrees F on an instant thermometer, and the thigh meat reads 165 degrees F (approximately 80-100 minutes). If the skin starts browning too quickly, you can reduce the oven temperature to 350 or 375 degrees, which will increase the cooking time but prevent the skin from darkening too much.
Remove from the oven and let rest, tented with foil, for 30 minutes before serving.
Notes
For brining, use approximately 2 teaspoons of kosher salt per pound of turkey.