Soak the mackerel in cold water with 1 tbsp vinegar for 10 minutes to remove the fishy smell and firm up the flesh.
Cut the radish into 2cm-thick slices.Slice the onion thinly and cut the green onion diagonally.
In a bowl, mix doenjang, soy sauce, cooking wine, tuna fish sauce, gochugaru, sugar, plum syrup, and minced garlic to make the sauce.
In a pot, place the radish and onion, then add half of the sauce. Pour in 250ml of water and simmer over medium-low heat for 20–30 minutes from the moment it starts boiling.
Add the mackerel and pour the remaining sauce on top. Add another 100ml of water and simmer over medium-low heat for 20 minutes.
To remove any remaining fishy smell, add 2 tbsp cooking wine and ginger powder while simmering.If the sauce reduces too much, add water in 50ml increments and continue simmering.If you have fresh ginger, adding a small amount instead of ginger powder is even better.
Add the sliced green onion and simmer for 1 more minute.