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1
Place the pork belly blocks in a pot with the rice-washing water. Bring to a boil and blanch for 10 minutes. Drain, rinse each block under cold running water, and pat dry. The surface should be clean and free of grey scum.
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2
Combine the dashi, sake, and mirin in a heavy pot or donabe. Bring to a gentle simmer. Add the pork blocks skin-side down. Lay an otoshibuta (drop lid) directly on the surface of the liquid. Maintain the heat at a bare simmer — 80–85 °C — for 1 hour. No bubbles should break the surface vigorously; a slow, occasional rise indicates the correct temperature.
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3
This is where the dish lives or dies: the braise must never boil. Add half the soy sauce and half the sugar. Replace the otoshibuta and continue to simmer at 80–85 °C for a further 30 minutes. Add the remaining soy sauce and sugar. Simmer for a final 30 minutes. A chopstick or skewer inserted into the thickest part of a block should meet no resistance, and the fat layer should appear translucent and trembling.
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4
Remove the pot from heat. Allow the pork to cool in its braising liquid, then refrigerate overnight. The fat will rise and solidify on the surface; skim and discard it. The braising liquid will have set to a loose jelly — this is the sign of full collagen conversion.
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5
Lift the pork blocks from the braising liquid. Transfer the liquid to a wide pan and reduce over medium heat until it reaches a sticky, glossy consistency that coats the back of a spoon. Return the pork blocks to the pan and baste continuously over medium heat, turning each block, until every surface carries a mahogany-coloured lacquer. The glaze should be fragrant, deeply sweet-savoury, and cling without pooling.
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6
Arrange one or two blocks per portion in a shallow bowl. Spoon remaining glaze over each block. Place blanched negi alongside and a small mound of karashi to one side. Serve immediately while the glaze is still glossy.