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1
Remove the seeds from the kabocha. Cut the flesh — skin intact — into pieces of approximately 5 cm × 5 cm. Do not reduce the pieces further: kabocha disintegrates in smaller cuts.
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2
Combine the dashi, mirin, soy sauce, and sugar in a wide, shallow pan. Arrange the kabocha pieces in a single layer, skin-side down. The broth should reach 60–70 % of the way up the sides of the pieces — no higher.
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3
Set an otoshi-buta (drop lid) directly on the kabocha. Bring the broth to a gentle simmer over medium heat. The drop lid keeps the pieces submerged and channels steam across the exposed surfaces, basting them continuously. Do not flip the pieces at any point: the skin-side-down orientation provides the structural support the flesh requires.
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4
This is where the dish lives or dies. Simmer, without stirring or turning, until the broth has reduced completely — no pooling liquid remains in the pan — and each piece is coated in a dry-but-glossy glaze. The timing demands attention: the liquid must exhaust itself at precisely the moment the kabocha reaches doneness. Excess liquid produces waterlogged flesh; insufficient reduction leaves the glaze unformed. Watch the pan: as the last of the liquid evaporates, the surface of each piece should take on a gentle lacquered sheen.
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5
Test the flesh by pressing a piece lightly — it should yield immediately but hold its shape and not collapse. The skin will be intact and slightly firmer than the interior.
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6
Remove from the heat. Scatter sesame seeds or kinako over the pieces if using. Serve alongside plain steamed rice, or allow to rest — kabocha nimono is arguably better the following day, once the broth has fully penetrated the floury interior.