1. In a large pot, bring the water, kosher salt, and brown sugar to a boil. Stir until the salt and sugar have completely dissolved.
2. Add the black peppercorns, allspice berries, coriander seeds, bay leaves, onion, and garlic to the pot. Allow the mixture to cool to room temperature.
3. Once cooled, pour the brine into a large container or brining bag, and add the cold water and apple cider vinegar (if using).
4. Submerge the turkey in the brine, ensuring it is fully covered. If needed, weigh it down with a plate.
5. Refrigerate the turkey in the brine for at least 12 hours, or up to 24 hours for maximum flavor.
6. Before roasting, remove the turkey from the brine, rinse it under cold water, and pat it dry. Let it come to room temperature before cooking.

Remember, patience is key here. Letting the turkey brine for a full day allows the flavors to deeply penetrate the meat, resulting in a juicy and flavorful bird.