Mix 300cc water and 1/2 tsp salt in a tray until dissolved. Submerge the mackerel fillet in the saltwater, cover with plastic wrap, and refrigerate for 3 hours. Remove the mackerel and pat dry with a paper towel.

Cut the mackerel in half and score the skin. Place the mackerel in a colander and pour boiling water over it until the surface turns white. Transfer to a bowl of cold water, clean the surface, and pat dry.

Julienne 2 slices of ginger and soak in water to make garnish (needle ginger). Drain and set aside.

In a frying pan, combine 200cc water, 2 tbsp sake, 2 tbsp mirin, 1 tbsp sugar, and the remaining ginger slices. Heat over medium heat until boiling, then add the mackerel. Spoon the sauce over the fish and cover with a drop lid. Simmer on low heat for 5 minutes.

Add 1.5 tbsp miso to the pan and dissolve. Cover with the drop lid again and simmer for another 10 minutes.