This is a classic, simple, everyday way of cooking fish with an amazing flavour that is being made in millions of Japanese households every day. With only 3 ingredients in the marinade (and no chopping!), it takes all of 30 seconds to prepare. Be careful to ensure you cook it over medium heat and no higher - because of the sugar in the mirin, if you cook it on too high a heat the sugar will burn before the inside has been cooked.