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1
Place the anago, skin and bones intact, in a wide, heavy pan. Add the sake, mirin, and light soy sauce. Pour over enough cold water to cover the fish entirely. Set over low heat and bring slowly to a bare simmer — small bubbles breaking at the surface, never a rolling boil. The collagen in the skin and bones begins to dissolve into the liquid at this stage.
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2
Maintain the lowest possible simmer throughout cooking. Aggressive boiling emulsifies the fat and proteins, producing a cloudy, opaque liquid that will not set to a clear amber jelly. The broth should remain translucent and just trembling. This is where the dish lives or dies: a boiled broth cannot be recovered.
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3
When the fish is just cooked through, lift it carefully from the broth. Remove and discard the bones; reserve the flesh. Do not allow the fish to sit in the residual heat of the broth, as it will continue to cook. The flesh should be tender and just yielding to light pressure.
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4
Test the set: spoon a small amount of the warm broth onto a cold plate and place in the freezer for five minutes. The broth should gel firmly when cold. If it remains liquid, return the broth to the heat and reduce by one third to concentrate the collagen, then test again.
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5
Arrange the reserved fish flesh in flat containers or moulds. Strain the broth over the fish to cover. Allow to cool to room temperature, then transfer to the refrigerator. The jelly is set when it holds a clean, trembling shape and the surface is clear amber with no cloudiness.
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6
To serve, turn out or slice the nikogori into neat portions directly from the flat container. Serve cold, with freshly grated wasabi alongside and a few drops of soy sauce. The jelly should be firm enough to hold its form on the plate yet dissolve cleanly on the palate.