Combine all the brine ingredients with only one gallon of water.
Bring to a boil and boil until all the salt is dissolved and flavors have melded about 8 minutes.
Pour into brining container, I use a 5-gallon bucket, add the turkey, and pour in enough extra water to cover the turkey completely.
Refrigerate at least 4 hours, up to overnight.
To Roast the Turkey
Preheat oven to 400 degrees F.
Remove turkey from brine, rinse thoroughly and pat dry.
Sprinkle with salt and pepper, rubbing it onto all parts of the turkey.
Prepare the roasting pan by laying the carrots, celery, onions, and herb sprigs in the bottom of the pan. Add 1 cup of the water.
Line the roasting rack in foil, spray with nonstick cooking spray, and poke 15-20 holes so that the air and liquid can still escape and circulate. Set aside.
Mix butter, chopped herbs, and citrus zest together. Gently separate the skin from the meat and work the butter mixture up in between the skin and meat wherever you can. Remember to do the front and back of the turkey.
Place quartered orange and lemon along with the garlic head inside the cavity of the turkey. Tie the legs together with cooking twine.
Place the turkey breast side DOWN in the roasting pan.
Roast for 1 hour, and then carefully flip the turkey over, breast side UP. Baste with liquid from the bottom of the roasting pan. Add more if needed.
Continue roasting until the breast meat reads 157 degrees F and the thigh meat reaches 185 degrees F (see post for full details on these temps). About 45-60 more minutes, basting every 15 minutes and checking liquid in the bottom of the pan, adding more if necessary.
Remove from oven and let rest 30 minutes.
For the Pan Gravy
Remove the roasting rack and vegetables from the roasting pan, leaving any turkey drippings behind.
On the stovetop, heat the drippings and mix in the flour.
Cook until lightly golden, about 2 minutes.
Whisk in the broth until smooth. Adjust consistency with more broth to your preference.
This recipe can easily accommodate a larger turkey. If your turkey is 20 pounds or larger, simply add 2 tablespoons of butter to the mixture and keep all other ingredients the same. A larger turkey will take longer to roast, about 10 minutes more per pound. As you check and baste, you will get a good idea for temperature and time. This recipe is not meant for a stuffed turkey. If you are stuffing your turkey, plan to lower the oven temperature to 350 degrees F and roast for much longer.