A type of flatfish, common in British waters.
en.m.wikipedia.org
It's not often that see a fish that I didn't already know about... but this is one of those rare treats.
So I can basically see that it's a cold water "flat fish". Apparently there's an American version that lives from Rhode Island and then north up into Canadian waters.
Down here in Florida we have a "flat fish" too. The flounder. But obviously it's a warm water fish. The meat is one of the finest you can put in your mouth. I imagine the cold water version might be able to get an even finer taste. For example... the cold water grouper we catch down here at extremely deep depths tastes better than any other grouper. Most people will never taste one though because it's not easy to get and nobody shares it with anyone outside of their family. It's just too amazing to let go of. We got a big yellow edge grouper a while back that I still dream about.
Out in our pacific northern waters there's a great big flat fish most people know about called the halibut. That meat is damn good too.
Flounder was my Grandmother's favorite fish so it has a special place in my heart. When I get a chance to eat it I always think of her.
Big Mahalos
@Londoner for posting this one Brudda!