Preheat the oven to 325 degrees F. Once preheated, place the oven thermometer in the oven to make sure your oven is reading an accurate temperature and adjust accordingly.
Remove the turkey innards and rinse the turkey well. Pat dry and place on a roasting rack set in a roasting pan. Salt the turkey cavity and squeeze the lemons inside the cavity. Stuff with the lemon rinds, herbs, and 2 onion halves. Add the remaining onion halves, all giblets and turkey parts, and 4 cups water to the bottom of the roasting pan. Rub the skin of the turkey with the butter and season with salt and pepper.
Place the turkey in the oven and cook until the internal temperature of the breast reaches 165 degrees F on a meat thermometer, 3 1/2 to 4 hours, basting with pan juices every 30 minutes. Cover the turkey with foil if the browning gets too dark. Remove the turkey when done and allow to rest 20 to 30 minutes before carving.