Combine the water, kosher salt, brown sugar, Cajun seasoning, garlic powder, onion powder, black pepper, bay leaves, thyme, lemon, and smashed garlic in a large stockpot. Make sure you have a big enough pot.
Once the brine has cooled, submerge the turkey in the brine in the same pot. Make sure it is completely covered in the brine. Put the lid on the pot or cover in plastic wrap or foil and refrigerate for at least 12-24 hours.
Preheat the oven to 325 degrees. Remove the turkey from the brine and pat it dry with paper towels. Rub the turkey with olive oil and the seasonings – coating evenly. Transfer the turkey to a roasting pan. Tuck the wings and tie the legs.
Roast, uncovered, for 3-4 hours (depending on weight) or until the internal temperature reads 165 degrees in the thickest part of the thigh. Baste occasionally. Let rest for 30 minutes before carving.