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1
Ensure the fish is same-day fresh. Fillet the aji, remove the skin and pin-bones, and reserve the belly and collar meat — these oil-rich cuts provide the binding fat that holds the mince. Dry the board thoroughly before beginning; a wet board dilutes the fish oils and prevents the aromatics from adhering.
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2
Lay the fillets flat on the dry board and begin chopping with a heavy, sharp knife using a rhythmic, consistent blade-strike motion — this is the tataki action that gives the dish its name and its texture. The blade must strike and separate, not press or grind. As the fish begins to open up after the first thirty seconds of chopping, scatter the green onion and shiso over the surface and continue chopping so that the aromatics are driven into the fish's own oils. Add the grated ginger in the same incremental manner, folding it into the mince with continued knife strokes. This is the step on which the dish lives or dies: too few strokes and the fish is unworked; too many and it becomes a smooth paste. Stop when the mince is coarse, slightly fibrous, and shows individual fish strands — the aromatics should be fully integrated and invisible as discrete pieces.
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3
If using umeboshi, tear the flesh of the pitted plums across the surface of the mince and work in with two or three further strokes only — enough to distribute, not to homogenise. The mince should remain visibly textured throughout.
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4
Season with miso or soy, adding less than instinct suggests. Work the seasoning in with the flat of the blade rather than further chopping strokes. Taste: the fish's own umami should be the primary flavour; the miso or soy is present as depth, not as a dominant note. The smell at this point should be clean ocean and sharp aromatics with no trace of oxidation or sourness.
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5
Serve immediately — over rice as a donburi, or as a standalone portion. The texture on the palate should be coarse and slightly fibrous. Any smoothness indicates the mince has been over-worked.