How to Make It
Put the turkey in a brining bag set in a tub or very large pot. In a large saucepan, combine the cider with the salt, 1 cup of the brown sugar and the rosemary, thyme and sage and bring to a simmer, stirring to dissolve the salt and sugar. Add 6 quarts of cold water to the brine and pour over the turkey. Add the ice to the brine and refrigerate the turkey overnight.
Preheat the oven to 450° and set a rack on the lowest shelf of the oven. Drain the turkey and pat dry. Discard the brine. Fill the turkey cavity with half of the onion, celery, carrot and garlic cloves; scatter the remaining vegetables in a large roasting pan. Set a V-shaped rack in the pan. Tie the turkey legs with butcher's twine and transfer the bird to the rack, breast side up. Add 2 cups of water to the pan and roast the turkey for 30 minutes.
Meanwhile, in a small saucepan, combine the remaining 1 cup of brown sugar with the 1 stick of butter and the bourbon and heat just until the sugar and butter melt.
Reduce the oven temperature to 350° and brush the turkey with some of the glaze. Continue roasting the turkey, brushing it every 15 minutes, for about 3 hours, until an instant-read thermometer inserted into the thigh registers 165°; add another 2 cups of water and tent the turkey with foil halfway through roasting. Transfer the turkey to a carving board and let rest for 30 minutes.
Strain the pan juices into a heatproof bowl and skim off the fat. (You should have about 1 cup.) Discard the vegetables.
In a large saucepan, heat the oil. Add the chopped turkey giblets and cook over moderate heat until lightly browned, about 5 minutes. Add the pearl onions and cook until lightly browned in spots, about 5 minutes longer. Add the turkey stock and the reserved turkey pan juices and bring to a boil, scraping up any browned bits from the bottom of the saucepan.
In a small bowl, mash the remaining 2 tablespoons of butter with the flour and whisk it into the gravy. Bring to a boil and simmer until the gravy thickens, about 5 minutes. Whisk in the roasted garlic. Carve the turkey and serve with the gravy.