A very tasty item that is usually an off menu dish at Japanese restaurants is hamachi kama, which is the neck of the hamachi and usually served grilled. The neck is saved after the sushi chef is done with the whole fish. Asians are not deterred by the work one has to go through to get to the luscious meat, which hides in the nooks and crannies of the collar bones. I know you can ask for it at Mizu.
I was intrigued by a listing for “tuna collar” at the Bristol, one of my favorite restaurants, and was pleasantly surprised to see that it was simply hamachi kama, but with tuna instead of hamachi. Served with a soy glaze and grilled, the portion was huge, indicating just how much bigger tuna is than yellowtail. The flavor was a bit richer than your average tuna steak, and better in my opinion. You also had to pick at the bones to get to all the meat, but it is worth it.