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1
Place the scored, dry-brined duck breasts skin-side down in a cold, dry pan. Set over a low flame and allow the fat to render slowly — do not raise the heat until the fat cap has largely melted away and the skin lies flat against the pan. Once substantial fat has pooled, raise the heat to medium-high and continue until the skin is crisp and golden. Flip and cook the flesh side briefly, targeting an internal temperature of 58–60 °C (pink, medium-rare centre). Remove and rest. Reserve every drop of rendered fat in the pan.
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2
In the fat remaining in the pan over medium-high heat, add the 5–6 cm negi sections. Sear without moving until the cut faces are deeply caramelised and the negi yield slightly under gentle pressure. Remove and set aside.
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3
Deglaze the pan with the sake, scraping up all Maillard fond. The liquid will smell of caramelised negi, duck fat, and toasted grain. Pour this deglazing liquid into the prepared soba tsuyu (dashi, soy sauce, mirin combined). Add all remaining rendered duck fat and pan juices. Bring to a minimum of 85 °C and hold; the broth must be at high heat so that the duck fat remains emulsified and aromatic.
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4
Slice the rested duck breast on a slight bias at 5 mm thickness. Cook and drain the soba noodles; divide among four warmed bowls.
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5
Arrange the sliced duck breast and caramelised negi sections over the noodles in each bowl. Ladle the boiling-hot duck-fat-enriched tsuyu broth over at the table. The broth must arrive at a vigorous simmer so that the fat holds in suspension and the surface shimmers uniformly.