Pour 4 cups of the cold water into a large stockpot.

Add kosher salt and brown sugar; heat over medium-high, stirring, until completely dissolved, about 3–4 minutes.

Stir in smashed garlic, peppercorns, thyme, rosemary, bay leaves, and the halved lemon (squeeze juice in, then drop halves).

Bring the mixture to a gentle simmer and cook for 3–5 minutes to release flavors.

Remove pot from heat. Add remaining 4 cups cold water and the ice cubes; stir until the brine is room temperature or colder.

Place prepared chicken (up to 4–6 lb) in a food-safe container or brining bag.

Pour cooled brine over the chicken, ensuring it is fully submerged; weigh down with a plate if necessary.

Cover and refrigerate 4–24 hours (2–4 hours for boneless pieces, 4–8 hours for bone-in parts, up to 24 hours for a whole bird).

Remove chicken from brine, discard liquid and aromatics, and briefly rinse under cold water.

Pat chicken completely dry with paper towels; proceed with your preferred seasoning and cooking method.