A #notsushi post, but it's about tuna (kind of).
One of best places to casually eat seasonal seafood is an izakaya. Even mid-level ones will have seasonal sashimi on the menu, and there may even be some surprises. These pictures are from Kane Masu near Kachidoki, which opens mid-afternoon and closes before 8pm. I guess their business is mostly focused on catching commuters before they grab the train home. Which kind of makes sense, since this area is pretty dead at night.
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These pictures are from a visit a few years ago, where both tuna head meat (noten) and rib scraps (nakaochi) were on the menu. Both uncommon cuts for sushi, but super tender and delicious. Served rustically, almost in a pile, with a chunk of wasabi. These were two of about 6 or 7 dishes that night, which included nikomi (beef stew with tendon). Many izakaya will have a pot of nikomi cooking away all day, and it's usually the "specialty" which means you should order a least a serving to try. It's like tasting the character of the shop.
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Years ago, Kane Masu became famous for serving raw wagyu "maki" stuffed with uni and that became a sort of insta-hype item, but it seems it is no longer on the menu anymore. Not that you'd be able to tell, as the menu is illegible even to some Japanese-speakers. I could probably make 5 or 6 more posts on this place, but this account isn't @izakayaaddict so I'll just say that this is definitely a spot worth visiting.