Fill a four gallon stockpot ⅔ full with water and bring to a boil over high heat. In a small bowl, combine all of the Cajun seasoning ingredients and set aside. To the stockpot, add potatoes, garlic, onion, 6 lemon halves, salt and half of the seasoning mix. Cover and bring to a boil. Reduce heat to medium-low and cook for 10 minutes.