In a large stockpot, combine 1 gallon of water, Kosher salt, brown sugar, black peppercorns, rosemary, thyme, smashed garlic, the quartered citrus, and bay leaves.
Place the pot over medium-high heat and bring the mixture to a simmer. Stir continuously until the salt and sugar are completely dissolved into the water (about 5-10 minutes).
Remove the pot from the heat and let it cool slightly at room temperature.
Pour the concentrated brine mixture into a large brining bucket, a massive stockpot, or a heavy-duty brining bag. Immediately add the 1 gallon of ice water to bring the temperature down rapidly. Do not proceed to the next step until the liquid is completely cold.
Carefully submerge your completely thawed, raw turkey into the cold brine, breast-side down. Make sure the bird is fully covered by the liquid. Seal the bag or cover the bucket.
Place in the refrigerator and let the turkey soak for 12 to 24 hours (a good rule of thumb is 1 hour per pound of turkey).
When ready to roast, remove the turkey from the liquid. Discard the brine entirely. Pat the turkey completely dry inside and out with paper towels before roasting to ensure a crispy skin.