One 18-pound turkey, wing tips, neck and giblets reserved
Kosher salt and freshly ground pepper
2 large onions, thinly sliced
2 large carrots, thinly sliced
2 large celery ribs, thinly sliced
4 garlic cloves, thickly sliced
2 large shallots, thickly sliced
12 large fresh sage leaves, plus additional sprigs for garnish
12 large fresh thyme sprigs, plus additional sprigs for garnish
2 bay leaves
5 tablespoons unsalted butter, softened
Turkey Stock and reserved turkey giblets
5 tablespoons all-purpose flour
1/2 cup semidry Riesling
Preheat the oven to 400°. Season the turkey cavity with salt and pepper and fill it with half of the onions, carrots, celery, garlic, and shallots along with the 12 sage leaves, 12 thyme sprigs, and the bay leaves. Tie the legs together with string, wrapping it around the tail to close the cavity.
Rub the turkey all over with 2 tablespoons of the butter and season with salt and pepper. Spread the remaining vegetables in a large roasting pan and set the turkey on top. Add 1 cup of the Turkey Stock to the pan. Roast the bird for 1 1/2 hours, basting generously and adding 1 more cup of stock to the pan after 1 hour.
Add another 1/2 cup of stock to the pan and cover the turkey breast loosely with foil. Reduce the oven temperature to 325°. Roast the turkey for 1 1/2 hours longer, or until an instant-read thermometer inserted in the inner thigh reaches 165°; baste often and add another 1/2 cup of stock to the pan after 1 hour. If the turkey isn't nicely browned, remove the foil for the last 30 minutes of cooking.
Turn the oven off and leave the turkey in for about 1 hour, or until the temperature of the inner thigh reaches 180°. Transfer the bird to a large platter, cover loosely with foil and let rest for up to 1 hour.
Meanwhile, strain the roasting juices into a medium saucepan; reserve the vegetables. Add the remaining 3 cups stock to the pan and bring to a simmer, skimming as necessary.
In a large nonreactive saucepan, melt the remaining 3 tablespoons butter. Add the reserved vegetables and stir in the flour. Cook the mixture over moderate heat, stirring frequently, until very thick and brown, about 5 minutes. Stir in the wine and cook, stirring frequently, until very thick again.
Gradually whisk the hot stock and roasting juices into the gravy base and simmer over low heat for 10 minutes, stirring often. Strain the gravy into a medium saucepan and simmer gently over low heat for about 15 minutes, skimming occasionally; you should have about 4 cups. Add stock if the gravy is too thick. Finely chop the reserved giblets that were cooked in the stock and stir them into the gravy; simmer for 5 more minutes, stirring occasionally. Season with salt and pepper and keep warm.
Discard the string and the vegetables from the turkey. Garnish the bird with the sage and thyme sprigs. Carve the turkey at the table and pass the gravy alongside.