Purchase frozen turkey 7 days before you plan to roast it. Place in the refrigerator and allow turkey to thaw for 3 days.
On the 4th day, remove turkey from packaging. Take neck and giblets out of the turkey cavity. Remove any excess fat and leftover pinfeathers. Wash the turkey inside out. Pat dry with paper towels. Sprinkle cavity with kosher salt.
Zest the lemon. After zesting, keep the lemon in the refrigerator.
Combine lemon zest, poultry seasoning (or rubbed sage), dried thyme, salt, and ground pepper in a small bowl.
Rub mixture all over the turkey.
Place turkey on a roasting rack in a deep pan breast side up and wrap with plastic wrap. Refrigerate for 2 days.
On the morning of roasting day, cut reserved lemon in half. Peel onion and cut in half. Slice each half of the onion but not all the way so that the slices are together.
Remove turkey from the refrigerator. Stuff one half onion and one half lemon in the turkey cavity. Tie the legs together with kitchen twine.
Turn the turkey over. Stuff remaining half onion and lemon into the neck cavity. Fold the neck skin over and secure it in place with toothpicks.
Place the turkey with breast side up on the rack. Tug the wings under the body. Return the turkey unwrapped to the refrigerator until time for roasting.
Preheat oven to 375°F (190°C). Brush melted butter all over the turkey. Pour 2 cups (480ml) water into the pan.
Transfer turkey to the preheated 375˚F (190˚C) oven and roast for 30 minutes. Reduce heat to 325˚F (165˚C) and continue to roast for another 2 to 2½ hours.
If the turkey is browning too quickly, tent it with aluminum foil.
According to the FDA, the turkey is safe when a food thermometer inserted into the innermost part of the thigh and wing and the thickest part of the breast reaches a temperature of 165°F (74°C).
Remove from oven and allow turkey to rest for 20 minutes before carving.