I'd love to know why this recipe specifically calls for a cast iron pan. I've only (accidentally) cooked tomatoes or other acidic things in a cast iron pan a couple of times, and had horrible results. Is it a matter of time? I've always used a well-seasoned pan, but the finished dish was positively bitter and metallic. So what gives? And couldn't the same result be achieved with another sort of pan? This soup sounds lovely, and I trust that it works, but I'd like to understand it a little better.