When your turkey goes into the oven, start the broth: Melt the butter in a large saucepan over medium-low heat. Add the onion and turkey neck and giblets; cook, stirring, until the giblets are browned, about 15 minutes. Add the chicken broth, herb sprigs and bay leaf; cover and simmer while the turkey roasts, about 2 hours. Strain the broth and keep warm; reserve the neck and giblets, if desired.
When your turkey is done, transfer it to a cutting board and pour all the pan drippings into a degreasing cup. Add 1/2 cup of the prepared broth to the roasting pan and scrape up the browned bits with a wooden spoon. (If the bits are stuck, put the pan over a low burner to loosen them.) Add the bits and liquid to the degreasing cup.
Let the fat rise to the top of the degreasing cup, then spoon off 1/2 cup fat and transfer to a large saucepan over medium heat. Make a roux: Sprinkle the flour into the pan and cook, stirring constantly with a wooden spoon, until the flour browns slightly, about 4 minutes.
Gradually add the hot broth to the roux, whisking constantly to prevent lumps. Bring to a boil, then reduce the heat to medium low. Pour the dark roasting juices from the degreasing cup into the gravy, discarding any remaining fat. If desired, chop the giblets and shred the neck meat; add to the gravy. Simmer, whisking occasionally, until the gravy thickens, about 10 minutes. Add the Worcestershire sauce and season with salt and pepper. Stir in the flavored butter, if desired.