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1
Place the kombu in the cold water and bring slowly to just below a simmer. Remove the kombu before the water reaches a full boil. Add the katsuobushi, bring briefly to a boil, then remove from heat. Allow to steep for 4 minutes, then strain through a fine sieve. The broth should be clear, pale amber, and carry a clean ocean-smoke scent.
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2
Return the strained dashi to a clean pot. Add the usukuchi soy sauce. Warm to just below a boil (approximately 80–85 °C). Add the sliced ofu and hold at this temperature. The broth should remain visibly pale and clean — a dark or murky broth signals the dish has moved away from the Nagoya character.
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3
Bring a large pot of unsalted water to a rolling boil. Add the kishimen noodles. Cook 8–10 minutes from the moment of adding, testing frequently from the 8-minute mark. The flat noodle is done when it is fully tender through its cross-section with a just-firm bite at the centre — the flat form is less forgiving than round noodles; a further 90 seconds beyond this point renders the noodle irretrievably soft. This is where the dish lives or dies.
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4
Drain the noodles immediately. Divide into four warmed deep bowls, allowing the noodles to settle flat and separated rather than bundled. Ladle the hot dashi broth generously over the noodles — kishimen requires more broth than an equivalent weight of round udon, as the flat surface absorbs and wets faster; err toward a full, deep pool of broth. The noodles should be fully submerged.
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5
Distribute the ofu pieces across each bowl. Scatter the sliced green onion. At the table, place a mound of fresh katsuobushi shavings directly on the noodle surface. Serve immediately: kishimen cools and becomes sticky more rapidly than round noodles, and the dish is correct only at near-boiling temperature (80 °C and above). The katsuobushi shavings should visibly tremble from the broth heat — this is the signal to eat.