Mirin-grilled refers to the grilling of semi-dried fish known as mirin-boshi. Mirin-boshi is made by splitting the fish, marinating it in a seasoning liquid made of soy sauce, sugar, and mirin, and then drying it. In Japanese cuisine, it's as common a preparation method as salting and drying fish fillets, with mackerel, sandfish, and other varieties being turned into mirin-boshi. Mirin is a traditional Japanese seasoning, a sweet sake made by fermenting glutinous rice and koji mold in shochu.