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1
Shred the used dashi kombu into fine strips of 2–3 cm length. Small, uniform pieces are functional: greater surface area relative to volume ensures even absorption of the simmering liquid and consistent preservation throughout.
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2
Place the shredded kombu in a small, heavy saucepan. Add the soy sauce, mirin, sake, and sugar. If using togarashi, add it whole at this stage. Set over a medium-low flame and bring to a gentle simmer, stirring once to dissolve the sugar.
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3
Continue simmering steadily over low heat, stirring occasionally. This is where the dish lives or dies: the simmering must not be interrupted until every trace of liquid has been absorbed and the kombu strips are lacquered in a dark, caramelised soy-sugar glaze. Any residual moisture will shorten shelf life and produce a wetter, fundamentally different product. The pan will begin to look nearly dry, and the strips will take on a deep mahogany sheen — this is the correct end point.
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4
Remove the togarashi if used. Spread the finished tsukudani on a flat tray or plate to cool completely before storing. The texture when pressed between fingertips should be firm and slightly tacky, without any yielding moisture.
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5
Taste a single strip on plain rice. The flavour must read as intensely savoury, sweet-soy concentrated, and slightly caramelised — a small quantity should transform the rice without overwhelming it. Adjust at this point only if the soy depth is absent (a few more drops of soy, returned briefly to heat) or if sweetness dominates without savoury support.