That's a pretty broad statement. Mackerel is very common in Japanese restaurants (and homes), either broiled or raw. I'm sure in other cultures' cuisines too -- Mediterranean, southeast Asian, northern Europe. But it is a strong-tasting fish, and the lack of diversity of fish available in the States, and the inexplicable refusal of many (Euro) Americans to eat fish at all means it's not well known outside specific communities here. I get it at my local Asian market, wrap it in foil, and grill it on the Weber. (Don't cook it inside unless you have a very strong extractor fan!)
LeFevre - "Pickled ginger which I made myself". Wow, so talented.
And the mandatory use of "umami". Enough with the overuse of that word.