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1
Place the kombu in the cold water in a clean pot. Bring slowly to just below a boil; remove the kombu. Bring the liquid to a full boil, add the katsuobushi, and remove from the heat immediately. Allow to steep for 2–3 minutes, then strain through a fine cloth without pressing. The dashi is ready when it is clear, golden-amber, and unmistakably aromatic.
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2
Return the strained dashi to the pot. Season with salt and shoyu. Taste: the broth should be gently savoury and clean, with no single element dominant. Season now; the egg cannot be seasoned effectively after it sets into ribbons.
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3
Dissolve the katakuriko thoroughly in 2 tablespoons of cold water — never hot water, which gelatinises potato starch on contact and produces irreversible lumps. Bring the seasoned dashi to a gentle simmer. Pour in the starch slurry in a slow stream, stirring constantly. The broth is sufficiently thickened when it turns slightly translucent and coats the back of a spoon with a soft, fluid body — not gelatinous, not gluey.
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4
This is where the dish lives or dies. Maintain the broth at a gentle simmer — not a rolling boil. Raise the beaten egg above the pot (30–40 cm) and pour it in a very thin, slow, controlled stream. Stir the broth in one direction only throughout; bidirectional stirring fragments the forming ribbons into small egg pieces. The egg must be loosely beaten with visible yellow-white streaks: these create the characteristic two-tone marbled ribbon suspended in the transparent broth. When the ribbons have set — silky, soft, and fully opaque — remove from the heat at once.
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5
Ladle immediately into warmed bowls. Place a single seasonal garnish — kinome, mitsuba, trefoil, or a slip of citrus zest — at the centre of the surface. The garnish should be the only visible colour accent against the amber-gold broth.