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1
Place the pork belly cubes skin-side down in a wide pot with the negi, ginger, and 2 litres of cold water. Bring to the boil, then blanch at a steady boil for 10 minutes. Drain and discard the blanching liquid and aromatics. Rinse each cube under cold running water, removing any grey scum. The surface of the meat should be clean and the water running off it clear.
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2
Return the pork to a clean, heavy pot just wide enough to hold the cubes in a single layer. Add the sake, mirin, soy sauce, sugar, and enough water to just cover. Bring to a bare simmer over medium heat, then immediately reduce to the lowest possible heat. A single lazy bubble rising every few seconds at the surface is correct. This is the step on which the dish lives or dies: maintain this gentle simmer without allowing the liquid to boil. A rolling boil will emulsify the fat into the braising liquid, producing a greasy, murky result, and will tighten the meat to an unacceptable toughness. Hold this simmer for a minimum of 1 hour 30 minutes to 2 hours.
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3
Test the pork at the 1 hour 30 minute mark by pressing a cube gently with a finger or the back of a spoon. The cube should yield with almost no resistance, the fat layer feeling as soft as set jelly. If it offers firmness, continue simmering and test every 15 minutes.
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4
Remove the pork cubes carefully and set aside. Raise the heat under the braising liquid to medium-high and reduce until the liquid thickens noticeably, stirring occasionally. When the liquid coats the back of a spoon and falls from it in slow, heavy drops rather than a stream — and carries a deep mahogany colour with a visible gloss — it has reduced sufficiently.
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5
Return the pork cubes to the reduced liquid over low heat. Turn each cube repeatedly to coat it in the lacquer. The surface of each cube should take on a deep, glossy, mahogany sheen. When the coating clings to the skin and fat without pooling at the base of the pot, the dish is ready to serve.