Blanch mackerel fillets in the boiling water and shock the fish in the cold water.

Cut naganegi vertically and remove the core. Julienne and soak in the water and drain.

Bring A to a boil in a pot. Place (1) and place a drop lid on top. Simmer on low heat for about 10 minutes. Remove the drop lid and simmer for another 5 minutes.

Serve on a plate and garnish with (2). Top with kinome or ground sansho if desired.

Recipe notes:
The drop lid (otoshibuta) is traditionally made with wood, but also comes in stainless steel, silicone, disposable parchment paper or food-grade foil alternatives.