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1
Rinse the Koshihikari rice under cold running water until the water runs clear. Drain thoroughly. Place the rinsed rice and the cold kombu dashi together in a heavy-bottomed pot — begin with cold liquid, not hot. The cold start is intentional: it allows the starch to hydrate gradually and contributes to the final creaminess.
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2
Set the pot over medium heat and bring slowly to a gentle simmer. Do not rush this stage. Once the surface shows a quiet, steady movement — small bubbles breaking at the edges, no rolling boil — reduce the heat to its absolute minimum. Set the lid slightly ajar to allow steam to escape and prevent boiling over.
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3
Cook at the lowest possible heat for 45 to 60 minutes. This is where the dish lives or dies: high heat will scorch the bottom before the surface cooks properly, destroying the porridge's character; only sustained, patient low heat allows the grains to break down fully and release their amylopectin into a unified, creamy mass. As the porridge approaches its final thickness, stir gently and constantly to prevent the bottom from catching and to develop creaminess throughout.
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4
Remove the pot from the heat. Replace the lid fully and allow the okayu to rest for several minutes. The rest period is not optional — it allows the porridge to achieve its final consistency. When ready, no individual grain structure should be visible; the surface should appear smooth, unified, and softly gleaming.
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5
Taste the okayu. Season with the smallest quantity of fine sea salt — the porridge is traditionally served lightly seasoned, with the accompaniments providing salt at the table. Ladle into warmed bowls. Place one piece of umeboshi in each bowl. Serve with fresh grated wasabi and quality soy sauce alongside, to be added by each diner in drops rather than quantities. The flavour should register as delicate, clean, and gently sweet, with a subtle mineral-savoury depth from the kombu dashi beneath.