How to Make It
In a large saucepan, combine the fennel, mustard and coriander seeds with the bay leaves, salt, sugar and 1 quart of the water. Bring to a boil, stirring to dissolve the salt and sugar. Transfer the mixture to a very large bowl or pot and add the remaining 7 quarts of cold water. Add the turkey, breast side down. Cover and refrigerate for at least 8 hours or overnight.
Preheat the oven to 450°. Drain and rinse the turkey and pat dry; discard the brine. Starting at the neck end of the bird, slip your hand between the skin and meat to loosen the turkey skin.
In a medium bowl, combine the 2 sticks of butter with the parsley, sage and thyme and season with pepper. Spoon 1/4 cup of the herb butter into a small bowl and stir in the flour; cover and chill. Spread the remaining herb butter all over and under the skin of the turkey and set it on a rack in a roasting pan. Add 2 cups of the stock to the pan and roast for 30 minutes. Lower the oven temperature to 325° and roast the turkey for about 2 1/2 hours longer, basting occasionally; the turkey is done when an instant-read thermometer inserted in the thigh registers 170°. Transfer the turkey to a cutting board and let rest for 30 minutes.
Meanwhile, in a large saucepan, melt the remaining 2 tablespoons of butter. Add the onion, carrot and celery and cook over moderate heat, stirring occasionally, until softened, about 5 minutes. Stir in the bread and cook for 1 minute. Add 2 quarts of the stock and bring to a boil. Simmer over moderate heat until the liquid is reduced to 3 cups, about 1 hour and 15 minutes. Whisk to break up the bread. Strain the stock through a fine sieve and return it to the saucepan. Whisk in the chilled reserved herb butter and bring the gravy to a boil. Cook, whisking, until thickened, about 5 minutes.
Pour off the fat from the roasting pan and place the pan over high heat. Add the remaining 2 cups of chicken stock and cook, scraping up any browned bits stuck to the pan, until boiling. Strain the pan drippings into the gravy and season with salt and pepper.
Carve the turkey and serve on a platter, passing the gravy at the table.