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1
Dissolve the salt in the measured water. Add the brine to the flour gradually, mixing with the fingertips until no dry flour remains and a shaggy mass forms. Turn onto a clean surface and work by hand for 10 minutes until the dough comes together into a unified, if still rough, mass. It will feel stiffer and drier than any standard pasta or bread dough — this is correct.
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2
Seal the dough inside a clean plastic bag, place it on a flat floor surface, and tread it evenly and repeatedly across the entire surface with the balls of the feet, using full body weight. Fold the sheet back on itself, re-seal, and tread again. Repeat this cycle — treading, folding, treading — for a total of four to five cycles until the dough is smooth, uniform, and presses back sharply against the finger. This is ashi-fumisoba, and it is here that the dish lives or dies: sustained mechanical gluten development beyond what hand-kneading alone achieves creates the characteristic koshi snap-back elasticity; insufficient treading produces an ordinary noodle.
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3
Wrap the dough tightly and rest at room temperature for a minimum of two hours. The dough relaxes but the developed gluten network remains intact. After resting, the surface should be perfectly smooth and the dough should yield slowly and evenly under a pressed thumb.
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4
Dust the rested dough with flour and roll to a sheet of uniform 3–4 mm thickness — no thinner. Dust the sheet, fold it loosely, and cut across the folds at consistent 3–4 mm intervals to produce square-section noodles. Unfurl and dust lightly to prevent sticking. A noodle held between two fingers should have a squared profile visible to the eye.
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5
Bring a minimum of 8 litres of water per 200 g of noodles to a rapid, rolling boil. Add the noodles without salting the water. Cook at a vigorous boil, stirring occasionally to separate, until the noodles are cooked through but retain marked bite resistance — this takes approximately 8–12 minutes depending on thickness; cut a noodle and examine the cross-section: it should be uniform with no white flour core visible, yet the noodle must spring back distinctly against the tooth. Overcooking destroys koshi irreversibly.
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6
For kake (hot) service: drain the noodles and briefly return them to a separate pot of hot water to reheat through before portioning into bowls. Meanwhile, combine the iriko and kombu with 1.2 litres of cold water, bring slowly to just below a boil, remove the kombu, then allow the iriko to simmer for five minutes. Strain; season with soy sauce and mirin. The broth should smell assertively marine and read a pale amber. Ladle hot broth over the noodles; the surface should steam visibly.
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7
Crack one raw egg into each bowl directly over the hot broth. Scatter sliced green onion over the surface. Serve at once: the raw egg is a garnish that cooks gently in the hot broth at the table. The bowl is ready when the green onion colour is vivid and the egg white has just begun to set at its edges.