How to Make It
Season the turkey inside and out with the kosher salt. Cover with plastic wrap and refrigerate overnight.
Uncover the turkey and let it return to room temperature. Stuff the cavity with the garlic, jalapeño, apple and 6 of the sage leaves. Transfer the turkey to a large roasting pan.
Preheat the oven to 350°. In a small saucepan, combine the cider with the butter and the remaining 6 sage leaves and cook over low heat until the butter has melted and the sage is fragrant, about 4 minutes. Dampen a 32-by-20-inch piece of cheesecloth with water and squeeze dry. Immerse the cheesecloth in the cider butter until the liquid is absorbed. Drape the soaked cheesecloth over the turkey breast and legs.
Roast the turkey for 30 minutes. Add 2 cups of water to the roasting pan and continue to roast for about 2 hours longer, rotating the pan a few times, until an instant-read thermometer inserted in the inner thigh registers 165°.
In a large saucepan, heat the vegetable oil. Add the turkey neck, wing tips and giblets, except for the liver, and season with salt and pepper. Cook over moderate heat, turning a few times, until nicely browned, about 12 minutes. Remove the turkey parts and reserve. Off the heat, stir the flour into the fat in the saucepan to make a paste. Gradually whisk in the cider until smooth, then whisk in the lager. Add 3 cups of water and bring to a boil over moderately high heat, whisking until thickened. Return the turkey parts to the saucepan and add the bay leaf. Cover and cook over low heat, whisking occasionally, until the gravy is flavorful, about 1 1/2 hours. Discard the turkey parts and bay leaf.
Carefully peel the cheesecloth off the turkey and discard. Transfer the turkey to a cutting board and let rest for 30 minutes. Pour the pan juices into a glass measuring cup and skim the fat. Add the pan juices to the gravy and bring to a simmer over moderate heat. Season the reserved liver with salt and pepper, add to the gravy and simmer until pink in the center, about 5 minutes. Remove the liver and cut it into small pieces.
In a blender, puree the liver with 1 cup of the gravy. Whisk the liver puree into the gravy and season with salt and pepper. Rewarm the gravy if necessary. Carve the turkey and serve with the gravy.