Gulf of Maine Redfish.

Thanks for the very thorough answer. I ended up not making it to that store on Friday, maybe next time they have them advertised.

They are moderately firm, white fish, sometimes called ocean perch, so you might look for recipes with that name. More generally, they are rockfish, so you could substitute them for any recipe calling for Pacific or other rockfish. You could also simply bake them.

If you're interested in their ecology, they grow very slowly in very deep waters. When people first started to fish them intensively, their populations dropped precipitously. Now, they're better managed, but you might want to be sure you're shopping from a reputable source. Monterey Bay Aquarium ranks the, as a good alternative rather than a good first choice, because the stocks are well managed, but fishing methods can harm other species.