If frozen, thaw in the refrigerator until fully defrosted, allowing about 24 hours per 4–5 pounds.
Remove the giblets and neck from the turkey and pat dry with paper towels; loosen the skin over the breasts and legs with your fingers.
Season all over with the kosher salt, including a light sprinkle inside the cavity; set on a rack over a rimmed sheet pan and refrigerate uncovered for 24–48 hours.
Remove from the refrigerator 1 hour before roasting; preheat the oven to 325°F and position a rack in the lower third.
Strip the leaves from 4 thyme sprigs, 2 rosemary sprigs, and 4 sage leaves; finely chop and mash with the unsalted butter and black pepper until smooth.
Gently loosen the skin and rub two-thirds of the butter mixture under the skin over the breasts and legs; spread the remainder over the skin.
Roughly chop the onion, carrot, and celery; toss with the olive oil and scatter in a large roasting pan, then pour in the low-sodium chicken stock.
Halve the lemon and the garlic head; place them in the cavity with the remaining herb sprigs, then tuck the wing tips underneath and tie the legs loosely with kitchen twine.
Set the bird breast-side up on the vegetables and transfer the pan to the oven.
Roast at 325°F until the thickest part of the thigh registers 160–165°F and the breast 155–160°F, 3–3 3/4 hours for a 12–14 pound bird; baste with pan juices after 1 hour and every 45 minutes, tenting loosely with foil if browning too quickly.
Transfer to a carving board and rest, lightly tented with foil, for 30–45 minutes so the juices redistribute.
Strain the pan juices into a saucepan, discarding solids; skim fat, warm the juices, carve, and serve with the warm pan juices.