In the clean food-grade brining container, combine the 7.5 L refrigerator-cold water, 300 g fine salt and 150 g brown sugar. Stir for 3–5 minutes, until no salt or sugar crystals remain on the bottom. Do not warm the brine.

Add the quartered lemon, rosemary, bay leaves and peppercorns. Check with the food thermometer that the liquid is 40°F or below before it touches the turkey.

Confirm the fully thawed turkey has no added salt or solution, and that the neck and giblets are removed from both cavities. Lower it carefully into the brine; the measured liquid must cover the bird in a snug container. Use a clean food-safe plate to keep it submerged if necessary. Cover and refrigerate at 40°F or below for 12 hours. Include this soak within the one-to-two-day refrigerator storage period for a fresh or fully refrigerator-thawed turkey.

Heat the oven to 325°F. Set a rack in the roasting pan. Carefully lift the turkey from the brine over the container so it drains without splashing. Discard all used brine and aromatics. Do not rinse the raw turkey. Pat the skin and cavities dry with paper towels, then wash hands and clean and sanitize surfaces and utensils that touched raw turkey or brine.

Set the turkey breast-side up on the rack with the cavity empty. Rub the skin with the 2 tablespoons olive oil; add no more salt. Tuck the wing tips under the bird and, if desired, loosely tie the legs with food-safe kitchen twine. Pour the 120 mL water into the pan below the rack. Wash hands again after handling the bird.

Roast the unstuffed 12-pound bird for approximately 3–3¾ hours, beginning temperature checks around 2¾ hours. If the skin browns too quickly, loosely cover the browned area with foil. Continue until a food thermometer reads at least 165°F in the thickest breast and the innermost thigh and wing, without touching bone. Golden skin and elapsed time do not establish doneness.

Rest for 20 minutes before carving. Refrigerate leftovers in shallow containers within 2 hours, or within 1 hour if the room or outdoor temperature is above 90°F. Use refrigerated cooked turkey within 3–4 days and reheat leftovers to 165°F.