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1
Place the adzuki beans in a small saucepan with the kombu and cover generously with cold water. Bring to the boil, drain immediately, and discard this first water — this blanch reduces bitterness from the skin compounds. Return the beans and kombu to the pan, cover again with fresh cold water, and bring back to a gentle simmer. Cook until the beans are just tender but entirely whole, with skins intact. The cook is done when a bean pressed between two fingers yields without resistance yet holds its shape without splitting.
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2
Drain the beans over a bowl, reserving every drop of the deep red-brown cooking liquid. Discard the kombu. Allow the liquid to cool to room temperature. The darker the liquid, the more vibrant the pinkish-red colour in the finished rice: this is the moment that determines the dish's colour.
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3
Drain the overnight-soaked glutinous rice and submerge it in the reserved adzuki cooking liquid. Leave to soak for a minimum of two hours, or until the grains have taken on a uniform pinkish-red stain throughout. Drain, again reserving the liquid.
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4
This is where the dish lives or dies. Spread the stained glutinous rice evenly across the perforated insert of a wooden seiro steamer lined with dampened cheesecloth. Scatter the whole adzuki beans across and through the rice. Set over a wok or pot of vigorously boiling water, cover tightly, and steam. Every ten to fifteen minutes, sprinkle the surface of the rice with reserved adzuki liquid to maintain colour and even moisture. Steam until the grains are fully translucent, uniformly glutinous, and no raw, chalky centre remains when a grain is bitten — the consequence of insufficient soaking or steaming is irreversible unevenness. The beans must remain whole throughout: a split bean is the one failure the dish cannot absorb.
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5
While the rice steams, combine the toasted black sesame seeds with the coarse salt in a small bowl and mix to make the goma shio. The ratio of sesame to salt should leave the sesame dominant, the salt present but not sharp on the nose.
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6
Turn the finished sekihan into lacquerware boxes or serving bowls. Serve immediately, with goma shio offered at the table for each diner to apply to taste. The dish is at its peak on the day it is made; the glutinous rice hardens perceptibly on refrigeration.