If you are using a frozen turkey, thaw it in the fridge for a few days before you brine it. Allow one day of thawing for every 5 pounds of meat. Do not brine a frozen turkey. (It won't absorb the brine).
Several hours or even the day before you want to brine the turkey, cook the brine. You need time to let it cool down completely; a warm brine will make the turkey absorb too much salt.
To make the brine, combine all ingredients in a very large pot and bring to a boil over high heat. Cook for 3-5 minutes or until the sugar and salt are dissolved. Turn off the heat. Let cool to room temperature, and refrigerate it if you are not using it right away.
Unwrap your turkey in a clean sink. Remove the neck, which is usually in the main cavity, and the giblets, which are usually in a bag in the other end. (Save for turkey gravy/turkey stock.) Rinse the turkey with cold water.
Place the turkey in a large pot or bucket and cover with cooled brine. You can do this in a turkey oven bag that is well sealed (You still need a large bucket or pot for it to sit in though.) If the turkey is not all the way covered, you can add a little more water to the brine. Mine wasn't completely covered and it was fine. Either way, be sure to flip the turkey 2/3 of the way through brining so that it brines evenly.
Refrigerate for 18-24 hours. Don't do it much longer than that or your turkey will get mushy and taste over seasoned.
Remove the turkey to a clean sink. Discard brine. Wash the turkey with cold water, or soak it in a plugged up sink for 15 minutes. You want to make sure to get all the brine off of it. The brine has already done it's work to tenderize the turkey meat, you don't need the salt on the outside.
Remove your turkey from the sink and pat dry with paper towels. (Clean your sink with bleach). Do your best patting it dry, you may need to keep grabbing more paper towels.
Wash your hands and make the sage butter. Add the 2 sticks butter to a medium bowl. Finely chop the sage on a cutting board. Crush and mince the garlic and place it on top of the chopped sage. Sprinkle with 1 1/2 teaspoons salt. Use the side of a chef's knife to smash the sage, garlic, and salt together until everything is incorporated. (See photos below)
Add the mixture to the bowl with the butter and combine.
Use your hands to separate the skin on the turkey breast from the meat. This is easier said than done. You will have to break apart some membrane-y feeling stuff and you have to be pretty forceful. Try not to tear the skin, of course. Try to get your hand as close up to the neck as you can.
Once you have gotten your hand under the skin, grab a fist full of sage butter and rub it under the skin. Use a lot, maybe close to half. Rub the rest of the outside and inside of the turkey with the sage butter until it is gone. Make sure you turn the turkey over and get the back, wings, legs, etc. The butter doesn't stick to the skin very well, but do your best.
Preheat your oven to 350 degrees F.
Now it's time to stuff the turkey with aromatics. You won't be eating the stuffing, it's just for flavoring. If your turkey is trussed with plastic, carefully remove the turkey legs from the plastic so that you have easier access to the cavity. Stuff in 1 roughly chopped carrot, the celery, apple, onion, thyme, rosemary and sage. Return the turkey legs to the plastic trussing, or tie together with kitchen twine.
Add the remaining carrots and 2 cups of water to the bottom of the roasting pan, underneath the rack.
Tuck the wings under the back of the turkey, like it's got its arms behind its head laying out on the beach. (Here’s a good demonstration.)
Get someone to hold open the turkey bag for you while you place it in.
Place the bagged turkey on a rack in a roasting pan, on top of the carrots. Seal the bag and cut a few slits in the top. Place the roasting pan on the lowest rack of your oven. Make sure your thermometer is visible through your oven window.
Set a timer for 2 hours and 15 minutes. After that, check your turkey every 5 minutes with a meat thermometer. It needs to be 165 degrees in the breast.
Remove the turkey from the oven and let it sit without opening the bag for at least 15-25 minutes. (This is a good time to use the freed oven for warming other dishes.)
Cut the bag open and use old oven mitts to transfer the hot turkey to a cutting board. Discard the aromatics. Carve the turkey. I like this tutorial from Alton Brown. Save the turkey carcass for homemade turkey stock. Then make this Creamy Turkey Rice Soup!