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1
Soak the red beans in cold water for a minimum of 12 hours, so the skins soften gradually from the outside in and the centres cook through without splitting during the final steam — the beans are ready when they have visibly swollen and the skins are taut but unbroken. Soak the black rice and rinsed white rice together in cold water for 30 minutes, so the black rice begins to bleed its colour evenly into the batch rather than streaking unevenly later. Drain all components thoroughly before combining.
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2
Place the barley in a small saucepan, cover with water, bring to a boil, and par-cook for 15 minutes, so the barley arrives at the final pot with a head start and finishes level with the rice and legumes rather than remaining chalky at the centre. Drain well. The barley is ready when the grains yield slightly at the centre under thumb pressure but remain firm — not chalky, not soft.
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3
Combine the drained white rice, black rice, par-cooked barley, soaked red beans, and mung beans in a heavy pot or rice cooker bowl. This is where the dish lives or dies: use approximately 480 ml of cold water for the combined grain-and-legume mass rather than the 600 ml you would use for plain rice of the same volume, because the legumes and grains carry absorbed water into the pot and release starch as they cook, leaving too much water to turn the whole batch into a wet, starchy mass — too little, and the bean centres stay hard.
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4
Cook over high heat until the water reaches a full boil, then reduce to the lowest possible flame, cover tightly, and cook for 20 minutes without lifting the lid, so the trapped steam does the final work of swelling each grain and bean from within rather than escaping and leaving the centres undercooked. Remove from heat and allow to steam, covered, for a further 10 minutes. The dish is done when no liquid pools at the edges and the grains have absorbed evenly — the black rice will have stained the batch a deep purple throughout.
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5
Fold through the perilla oil and scatter the toasted sesame seeds. Serve immediately, so the perilla's faint nuttiness lifts on the heat of the rice rather than settling flat as the bowl cools — the smell of toasted grain and perilla rising together signals the rice is ready for the table.