Add a gallon of water, kosher salt, and brown sugar to a large pot. Bring to boil then reduce heat to simmer. Whisk until sugar and salt dissolve.
Add the rest of the water to the pot, as well as the white onion, bay leaves, rosemary, crushed garlic, black peppercorns, orange zest, and lemon zest. Combine well and leave the mixture to cool down to room temperature.
While the brine is cooling, line your brining bucket (or chosen container) with a brining bag
Once the brine has reached room temperature, place your turkey in the brining bag and carefully pour the brining solution over the top. Seal the brining bag or cover with a lid. Leave the turkey to brine in the solution in the fridge or an iced cool box for 24 hours.
Place a wire rack over a baking tray. Carefully remove your turkey from the brine solution and sit it on a wire rack to allow any extra juices to drain away. Pat the turkey dry with paper towels (this helps to create a crispy skin when cooked) and refrigerate the turkey until it is time for it to be smoked.