Two or three days before you plan to roast the turkey, combine 3 tablespoons salt, the rosemary and lemon zest. Wash the turkey inside and out, drain it well and pat it dry with paper towels. Sprinkle 1 tablespoon of the salt mixture in the cavity of the turkey and rub the rest on the skin, including under the wings and legs. Place the turkey in a shallow dish to catch any drips and wrap the whole dish tightly with plastic wrap. Refrigerate for one or two days. The day before you plan to roast the turkey, remove the plastic wrap and leave it in the fridge. The skin will dry out and turn a little translucent.
Preheat the oven to 450 degrees F. Be sure your oven is very clean!
Place the onion, lemon and thyme in the cavity. Tie the legs together with kitchen string and tie the wings close to the body. Brush the turkey with the butter and sprinkle it with salt and pepper.
Roast the turkey for 45 minutes, placing it in the oven legs first. Lower the temperature to 325 degrees F and roast it for about another hour, until 165 degrees F for the breast and 180 degrees F in the thigh on an instant-read thermometer. Remove from the oven, cover the turkey tightly with aluminum foil and allow it to rest for 20 to 30 minutes. Carve and serve with the pan juices.