If you haven't had gumbo made in a cast iron pot, then you just haven't lived yet. There is a reason grandma insisted on using that pot! When you make a roux for your soups and gumbos in a cast iron pot, the cooking is more consistent. The cast iron pot acts as a heat conductor. This makes the flour and oil cook more evenly. This is also the same for when you brown your onions, bell peppers and celery (the holy trinity).