heavy metal square pan with a uniform enamled surface inside and out with a very tight fitting lid. I bought it years ago, and I don't think they carry it anymore. I use it on the grill all the time and it still looks new!
Thanks, Richard, I may try my plain cast iron--it's well seasoned and one bottle of wine probably
won't hurt it. (And a little iron won't hurt us.) I just bought a big new oval Le Creuset casserole at an outlet store but I'm not willing to put that on an open fire just yet.
I love your idea of cooking the mussels and then using the liquid for everything else.