Two or three days before you plan to roast the turkey, combine 3 tablespoons salt, the minced thyme and lemon zest. Wash the turkey inside and out, drain it well and pat it all over 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 just large enough to hold it and wrap the whole dish tightly with plastic wrap. Refrigerate for 1 or 2 days. The day before you plan to roast the turkey, remove the plastic wrap and leave the turkey in the fridge. The skin will dry out and turn a little translucent.
Preheat the oven to 325 degrees F. Put the turkey in a large roasting pan, discarding any juices in the dish. Place the onion, lemon and thyme sprigs 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 generously with salt and pepper.
Roast the turkey for 2 to 2 1/4 hours, until the breast meat registers 165 degrees F on an instant-read thermometer (put the thermometer in sideways). Remove from the oven and place the turkey on a carving board. Cut off the legs and thighs and put them back into the roasting pan, covering the breast and carcass tightly with aluminum foil. Place the roasting pan back in the oven for 15 to 20 minutes, until the dark meat registers 180 degrees F. Remove the dark meat to the carving board with the turkey, cover it tightly with aluminum foil, and allow it to rest at room temperature for 15 minutes.
Pour a 1/4-inch layer of the gravy into a large (12-by-16-inch) ovenproof serving platter (make sure it's ovenproof!). Carve the turkey and arrange it 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.
For the gravy:
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.
Make it ahead: Roast and slice the turkey and assemble it on the gravy. Cover and leave at room temperature for up to 1 hour before reheating in the oven. Make the gravy base and refrigerate for up to a week or freeze for up to 3 months.