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1
Place the cooked rice in a sieve or bowl. Pour warm water over it, turning the grains gently with the fingers for 20–30 seconds, until the rinse water runs only slightly cloudy. Drain well. The grains must feel separate, not clumped — this removal of surface starch is the condition that allows the nabe broth to remain clear and light rather than gluey.
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2
Bring the strained nabe broth to a gentle simmer in the donabe or nabe pot over medium heat. Taste it: the broth must already carry the full, concentrated umami of the meal's cooking — zōsui is entirely dependent on what the broth already holds. Adjust seasoning at this point if necessary. This is where the dish lives or dies: a poor or thin broth produces a poor zōsui, and nothing added at the rice stage will correct it.
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3
Add the drained rice to the simmering broth. Distribute it evenly across the pot. Reduce the heat so the surface barely moves. Simmer gently for 5–8 minutes without stirring vigorously. The rice is ready when the grains have visibly swollen and become tender throughout, yet remain distinct — each grain surrounded by broth, not merged into a mass. Do not allow a vigorous boil at any point; this breaks the already-cooked grains into mush.
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4
If using the tamago-toji: ensure the surface of the zōsui is at a gentle simmer. Drizzle the room-temperature beaten egg in a slow, thin, circular stream across the surface while keeping the pot still for 20 seconds, then draw a single slow pass with a spoon. The egg is done when it has just set into pale, silken ribbons — opaque but not scrambled, still trembling slightly at the surface.
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5
Remove the pot from the heat. Scatter the sliced green onion and mitsuba over the surface. The residual heat will wilt the mitsuba to a soft-herb finish without cooking it further. Crumble nori over each bowl at the moment of serving so it retains its distinct texture rather than dissolving into the broth.