In a large, heavy-duty stockpot, combine the fresh apple cider, Kosher salt, brown sugar, cinnamon sticks, allspice berries, black peppercorns, orange slices, and fresh rosemary.
Place the pot over medium-high heat. Bring the aromatic liquid to a gentle simmer, stirring frequently until the Kosher salt and brown sugar have completely dissolved into the cider (about 5-10 minutes).
Remove the pot from the heat and allow the spiced cider concentrate to cool down slightly at room temperature.
Pour the liquid into your designated brining container (a massive pot, a brining bucket, or a heavy-duty brining bag). Immediately add the gallon of ice water to drop the temperature fast. Wait until the liquid is entirely ice-cold before proceeding.
Submerge your fully thawed, raw turkey into the chilled liquid, breast-side down, ensuring it is completely covered by the liquid.
Transfer to the refrigerator and let the bird soak for 12 to 24 hours to absorb all those wonderful autumnal flavors.
Remove the turkey, discard the liquid entirely, and pat the bird incredibly dry inside and out with paper towels before roasting to ensure a perfectly crisp skin.