To make the stock, sprinkle the snapper all over with salt and refrigerate for 20 minutes.
Bring water to the boil in a large pan and add the fish and 4 of the parsley stems, bruised.
Simmer gently for 30 minutes then strain through a fine sieve into another pot to make clear stock. Discard the bones.
Add soy sauce and mirin to the stock and simmer for around 5 minutes. Remove from the heat and add the ginger juice.
Pour some of the stock into a pan over medium heat and gently poach the fillet of snapper until it can be separated into flakes when a fork is inserted. Lift the fish out onto a plate and carefully separate into small chunks.
Heat the cubes of tofu in the same stock, and when heated, strain the stock back into the pan with the rest of the stock.
Place a slice of tofu and one quarter of the fish into each of four serving bowls, and pour the clear soup over.