In a large, heavy-duty stockpot, combine 1 gallon of water, Kosher salt, brown sugar, sliced jalapeños, crushed red pepper flakes, cayenne pepper, smashed garlic, and black peppercorns.
Place the pot over medium-high heat. Bring the fiery mixture to a gentle simmer, stirring frequently until the Kosher salt and brown sugar have completely dissolved into the water (about 5-10 minutes).
Remove the pot from the heat. Let the peppers and spices steep in the hot liquid as it cools slightly at room temperature, releasing their intense capsicum oils into the water.
Pour the spicy concentrate into your designated brining container (a massive pot or a heavy-duty brining bag). Immediately pour in the 1 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. Make sure the entire bird is fully covered.
Transfer to the refrigerator and let the bird soak for 12 to 24 hours to absorb all that moisture and spicy flavor.
Remove the turkey from the liquid and discard the brine entirely. Pat the bird incredibly dry inside and out with paper towels before roasting to guarantee a perfectly crisp, zesty skin.