Make brine (can be done up to 5 days ahead). Bring water to boil in a large saucepan or stockpot. (Note: If you have a stockpot that is large enough to hold the turkey and the brine, you can prepare the brine right in that pot.) When water is boiling, remove it from heat and stir in salt, lemon zest, brown sugar, thyme sprigs, and sage leaves. Let brine cool completely at room temperature or in the refrigerator. (This is very important. Pouring warm water over the turkey can prevent the turkey from staying at a safe temperature.)
12 hours before roasting: Take the fully defrosted turkey and rinse it well under cool tap water. Remove the neck from the cavity of the turkey and the bag of giblets (reserve for making gravy).
Combine turkey and brine in a large pot or sturdy food-safe bag. (The turkey should be fully covered in brine.) Return turkey to the refrigerator and let soak for 12 hours.
1 hour before roasting: About an hour before roasting, drain all of the brine off the turkey and rinse the turkey well to remove excess salt.
Place turkey breast-side up in a roasting pan (see note). Pat the skin dry with paper towels. Leave the turkey on the counter to allow it to come to room temperature.
Pour stock or water in the bottom of the roasting pan. If you’d like to add more flavor to the pan drippings, you can add some chopped onions, celery, carrots, and / or apples to the roasting pan.
Arrange oven rack so that one rack is in the lower third of the oven.
Preheat oven to 425°F / 218°C.
Just before roasting, loosen the skin from the breast and legs of the turkey to create some space for the butter. (Note: I like to use a spoon to help gently separate the skin.)
Rub softened butter evenly under the skin of the turkey.
Place lemon halves inside the cavity.
Tie / truss the legs of the turkey together (optional).
Transfer the turkey to the lower rack in the oven and roast for 90 minutes to 2 1/2 hours. Start checking the turkey on the lower end of the cook time and continue checking until the thickest part of the breast registers 160°F / 71°C and the thickest part of the thigh registers 165°F / 73°C.) If the breast is cooking much faster than the thigh (or if the skin on the breast gets more brown than you’d like it to be, lightly cover that part of the turkey with a piece of foil.)
Once turkey is done transfer it to a cutting board and let rest for 30 to 40 minutes (you can tent it with foil to keep it warm).
Slice and serve.