When choosing a turkey size, estimate about 1 to 1 1/2 pounds per person. Also try to buy a fresh (not frozen) turkey without additives since most frozen turkeys and some fresh are injected with a saline solution. This is not a good thing. Injected birds generally lack flavor and can have a mushy texture.
Thaw the turkey. It takes approximately 4-5 days for a 20-pound turkey to fully defrost. 1 to 2 days before cooking, take turkey out of the bag and place in a roasting pan. Pat dry with paper towels and then rub kosher salt all over, getting under the skin wherever possible. Use about 1 Tbsp salt per 4 lbs of turkey. If it's less than a day before roasting, place uncovered in fridge. If more than a day, drape loosely with plastic wrap til a day before roasting, then remove.
2-3 hours before roasting, take turkey out of fridge and let come to room temperature on the counter.
Place oven rack in lowest position and preheat oven to 350 degrees F.
Melt the butter in a small saucepan or microwave it in a covered dish. Add the zest and juice of the lemon and 1 teaspoon of thyme leaves to the butter mixture. Set aside.
Take the giblets out of the turkey and remove any excess fat and leftover pinfeathers and pat the outside dry. Place the turkey in a large roasting pan, breast up. Liberally salt and pepper the inside of the turkey cavity. Stuff the cavity with the bunch of thyme, halved lemon, quartered onion, and the garlic. Brush the outside of the turkey with the butter mixture and sprinkle with salt and pepper. Tie the legs together with string and tuck the wing tips under the body of the turkey or wrap in aluminum foil to keep from over-browning. Tightly cover breast area with foil, leaving wings, thighs, and drumsticks exposed.
Baste turkey with pan drippings every 45 minutes. If pan drippings have evaporated into a glaze, add some chicken stock.
Plan to roast for about 15 minutes per pound. For an unstuffed turkey between 10 to 18 pounds, it takes about 3 to 3-1/2 hours, or until the juices run clear when you cut between the leg and the thigh, and a thermometer inserted into fleshy part of thigh (close to but not touching bone) registers 180°F for full dark meat flavor, or 170F for a more moist turkey.
Remove the turkey to a cutting board and cover with aluminum foil; let rest for 20 minutes at least.
