Salt-Broiled Spanish Mackerel with Heirloom Tomatoes, Hijiki, and Sweet Onion

For this low-maintenance late-summer dish, I was inspired by the Spanish mackerels I’ve had in Japanese izakaya restaurants, which are most easily compared to pubs. The focus is on drinking, but there’s always some simple, delicious food served. On many occasions at such establishments, I’ve happily eaten a broiled piece of the fish placed beneath a salad of raw onions or beside some grated daikon and a side of ponzu sauce. The sweet sharpness of the former cuts the mackerel’s oiliness. The