This is the classic grilled yellowtail collar you see in sushi restaurants. Obviously you want a yellowtail collar, but I offer some substitutions above.
Mix the marinade ingredients together and put them, along with the yellowtail collars, into a heavy plastic bag or lidded container. Marinate overnight or up to 1 day. If the collars are not submerged, turn them periodically so they get good contact with the marinade.
The next day, pour the marinade into a small pot and bring it to a boil. Reduce it by half, and set it aside.
Pat the collars dry with paper towels and coat with a film of sesame oil. Get your grill nice and hot, and clean the grates.
Grill your collars over high heat, basting with the reduced marinade, for about 10 to 20 minutes, depending on how large your collars are and how hot your fire is. You want them fully cooked and a little charred. Serve with steamed rice and a salad.