Remove the neck and giblets from the turkey cavities. Trim off excess fat and remove any remaining pinfeathers.
Rinse the turkey thoroughly with cold water, both inside and outside, then dry it completely with paper towels. Removing surface moisture helps the skin become crisp during roasting.
Place the kosher salt, black pepper, dried sage, rosemary, and dried thyme in a small bowl.
Stir thoroughly until all of the herbs and seasonings are evenly incorporated.
Heat the oven to 375°F (190°C).
Set the prepared turkey breast-side up on a roasting rack positioned inside a deep roasting pan.
Spread the olive oil evenly over the entire exterior of the turkey, making sure all areas of the skin are well coated.
Place the quartered onion inside the neck cavity. Fold the loose neck skin over the onion to help hold it securely in place.
Arrange the fresh thyme sprigs inside the main body cavity, then scatter a portion of the prepared seasoning mixture inside the cavity.
Press the remaining dry rub evenly over the outside of the turkey so the entire surface is thoroughly seasoned.
Fold the wing tips underneath the turkey to protect them from excessive browning.
Secure the legs together with kitchen twine unless the turkey already has an attached leg holder.
Place the turkey in the preheated 375°F (190°C) oven and roast undisturbed for 30 minutes without opening the oven door.
After the initial 30 minutes, lower the oven temperature to 325°F (165°C).
Continue roasting for an additional 2 to 2½ hours. If desired, spoon the accumulated pan juices over the turkey once or twice during this stage to help keep the meat moist.
Check doneness with a food thermometer inserted into the innermost portion of the thigh and wing as well as the thickest section of the breast. The turkey is fully cooked when these areas reach an internal temperature of 165°F (74°C).
Remove the roasted turkey from the oven and allow it to rest uncovered for 20 minutes before carving.
The resting period gives the juices time to redistribute throughout the meat and helps produce cleaner slices.