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1
Blanch the pork and chicken bones in rapidly boiling water for 5 minutes. Drain, rinse under cold running water until the water runs clear, and discard the blanching liquid. Transfer the cleaned bones to a large stockpot, cover with 2 litres of cold water, and bring slowly to the point where the surface trembles but does not roll. Skim continuously for the first 30 minutes, removing all grey scum and fat that rises. Once the surface is clean, reduce to the barest simmer — individual bubbles rising, not a rolling boil. The broth is ready when it is fully transparent, pale gold, and smells of clean pork-chicken sweetness rather than raw bone.
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2
After the initial skimming, add the strained niboshi dashi to the simmering broth. Continue at a bare simmer, uncovered, for a further 2–3 hours. At no point allow the broth to boil actively — cloudiness from agitation cannot be corrected. The broth is done when a ladle held beneath the surface reveals the bottom of the pot through the liquid, and the flavour is a clean, light pork-chicken sweetness with a faint oceanic note from the niboshi. Strain through a fine-mesh cloth. Reserve hot.
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3
Combine the high-gluten flour and salt. Add cold water incrementally to achieve a hydration of 28–32% — the dough will feel noticeably stiffer and drier than standard pasta dough. Work until cohesive, then knead firmly for 10 minutes. Wrap and rest at room temperature for 30 minutes. The dough should feel dense and resistant to pressure, not supple — it must hold against the force of the bamboo pole.
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4
Divide the rested dough into portions. Flatten each portion by hand into a rough rectangle and drape over a long bamboo pole. Using the sao-uchi technique, press the pole down rhythmically against the dough with controlled body weight, rolling and re-draping as the sheet extends. Work progressively until the dough sheet is even and extremely thin — approximately 1.0–1.5 mm. The sheet should appear slightly translucent when held to the light. Rest the stretched sheets, covered, for 15 minutes to allow the gluten to relax before cutting. Cut into thin noodles of uniform width. The noodles should spring back slightly when pressed with a fingertip.
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5
Roll the pork belly tightly and tie at intervals. Place in a snug pot with the soy sauce, mirin, sugar, and enough water to nearly submerge. Bring to a simmer, cover, and braise gently, turning occasionally, until a skewer meets no resistance and the exterior is a deep, lacquered amber. Remove, cool, and refrigerate. Before service, slice 2–3 mm thick — thin enough that the fat melts into the broth without imposing. The slices should be translucent at the edges.
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6
Cook the sao-uchi noodles in a large pot of unsalted boiling water, stirring to prevent clumping. The noodles are thin and will cook rapidly. They are done when they are tender with a definite elastic spring — slightly chewy at the centre, not floury. Drain immediately.
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7
Warm four wide, shallow bowls. Place 15 ml of usukuchi tare in each. Ladle the broth — verified at 85–90°C — over the tare and stir briefly to combine. Taste: the broth should be gently savoury, light, and clean, with no single element forward. Arrange the drained noodles in the centre, lay the chashu slices to one side so that the fat meets the hot broth, and add menma, kamaboko, and negi. Serve immediately — the thin broth cools faster than heavier styles and must reach the table at temperature.