Combine water, kosher salt, brown sugar, Cajun seasoning, garlic powder, onion powder, black pepper, bay leaves, thyme, lemon, and smashed garlic in a large pot. Bring the mixture to a boil, remove it from heat and let it cool completely.

Once cooled, submerge the turkey in the brine, ensuring it's fully covered. Refrigerate for 12-24 hours, turning the turkey occasionally if needed.

Preheat the oven to 325°F (165°C). Remove the turkey from the brine and pat dry with paper towels.

Rub the turkey with olive oil and sprinkle with Cajun seasoning, covering the skin evenly.

Place turkey on a roasting rack in a large roasting pan, tucking the wings under and tying the legs with kitchen twine.

Roast the turkey for 3-4 hours, or until the internal temperature reaches 165°F (74°C) in the thickest part of the thigh. Baste occasionally with the pan juices.

Let the turkey rest for 20-30 minutes before carving.