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 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 the turkey and arrange it on a platter. Cover with plastic wrap or aluminum foil and allow the turkey to sit at room temperature for up to 2 hours.
To reheat, pour a 1/4-inch layer of the gravy into a large (12 × 16-inch) ovenproof serving platter (make sure it's ovenproof!). Arrange the turkey artfully on top of the gravy. Place the platter uncovered into the oven for 15 to 30 minutes, until the turkey is very hot. Serve hot with extra gravy on the side.
Make-Ahead Turkey Gravy with Onions & Sage:
Melt the butter in a large saucepan over medium heat. Add the onion and garlic and saute, stirring often, for 15 to 20 minutes, until the onion becomes browned and starts to caramelize. Sprinkle on the flour and cook, stirring constantly, for 1 1/2 minutes. Stir in the chicken stock, Cognac, sage leaves, bay leaves, 2 teaspoons salt (depending on the saltiness of the chicken stock), and 1 teaspoon pepper. Bring to a boil, lower the heat, and simmer for 20 minutes, stirring occasionally. Set aside at room temperature for 1 hour and strain, pressing the solids lightly and then discarding them. Refrigerate until ready to use.
After the turkey is cooked, remove it to a carving board to rest while you finish the gravy. Place the roasting pan on the stovetop over medium heat and add the wine. Bring to a boil, lower the heat, and simmer for 2 minutes, stirring and scraping up all the bits clinging to the bottom of the pan. Slowly whisk the gravy base into the pan. Simmer for about 5 minutes, until the gravy is smooth and slightly thickened. Taste for seasonings and serve hot.