「PREP」Put the mackerel in a strainer and pour some hot water over it to remove the slightly fishy smell. Slice the ginger.
Mix all the sauce ingredients and put in a saucepan. Cook over medium heat until warm.
Add the fish to the saucepan skin side down, and cook over medium heat for five minutes, constantly covering the surface with sauce.
Remove from heat and set aside, leaving the fish to soak for 30 minutes to an hour.* Gently heat it up again in the saucepan before serving (but don’t boil).
Notes
*Giving the fish some resting time allows it to fully absorb the flavours of the miso sauce. You can cook up some side dishes while it soaks. If you don’t have time and skip this step, it’s not the end of the world, but you will be missing out!