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1
Soak the black beans in cold water overnight, the glutinous millet in cold water for 2 hours, and the short-grain rice in cold water for 30 minutes, draining each separately so each grain has absorbed precisely the moisture it needs without borrowing time from another. The black beans are ready when they have swelled visibly and the skins show no wrinkling — a wrinkled skin means the bean will tighten and resist the steamer rather than cook through evenly.
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2
Place the adzuki beans in a small saucepan with double their volume of cold water, bring to a simmer, and cook for 30 minutes — starting them in cold rather than boiling water lets the skins soften at the same rate as the interior, so they stay intact rather than burst and lose their pigment into foam. The beans are done when they are just tender to a fingernail press but hold their shape without splitting. Reserve the cooking liquid, since it carries the purple-red pigment that will colour the rice throughout.
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3
Combine the drained short-grain rice, drained glutinous millet, sorghum, drained black beans, and the par-cooked adzuki beans with enough of the reserved adzuki cooking liquid and fresh water to moisten all grains evenly, so the pigment reaches every grain before heat sets it in place. Transfer to a steamer basket lined with a dampened cloth — the damp cloth holds fine grains like millet that would otherwise fall through while still allowing steam to pass freely up through the mass. Steam over vigorously boiling water for 30–35 minutes; the ogokbap is done when every grain is fully tender, the rice is cohesive but not mushy, and the surface shows an even reddish hue from the adzuki pigment distributed throughout, with no dry white patches of uncoloured rice remaining.
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4
Remove the steamer from the heat and turn the ogokbap onto a serving vessel, working gently so the beans hold their shape and the colour remains distributed rather than smeared. The colour distribution should be visible throughout — white rice tinted red, yellow millet, red-brown sorghum, and purple-black beans dispersed evenly, not clustered — and the aroma earthy and clean, with no single grain dominant. No salt is added; the dish is served pure alongside salty banchan.