Mategai are razor clams – these kind are not so common to find in Kanagawa actually. I’d never cooked them before but the fish market lady recommended sauteeing in olive oil with togarashi (chili). I did after washing them for some time to remove all the sand. I think they might work better in a broth or nabe though.
On the side a fresh wakame and wasabi green salad. The renkon was boiled in vinegar and water until slightly soft but still crunchy. Dressed with a soy sauce and vinegar dressing.