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1
Place the pork belly cubes in a pot and cover with cold water. Add the sliced ginger and negi lengths. Bring to a boil, then reduce to a steady simmer and hold for 30 to 60 minutes. The purpose is twofold: excess fat purges into the water and the protein structure tightens. The water will be visibly cloudy and fat-laden — this is correct. Drain; discard the liquid, ginger, and negi.
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2
Return the drained pork to the cleaned pot. Combine the soy sauce, mirin, sake, and sugar in the ratio 1 : 1 : 1 : 0.5 and pour over the pork. Add enough water to just cover. Place an otoshi-buta (drop lid) directly on the surface of the liquid; this keeps the pork submerged and the broth in gentle circulation. Bring to a bare simmer — the liquid should tremble, not boil.
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3
This is where the dish lives or dies. Maintain the barest possible simmer — a full boil will break the cubes apart and render the fat grainy rather than unctuous. Braise for a minimum of 2 to 3 hours, checking periodically. The dish is ready to proceed when the fat layer is completely translucent, jiggles freely when the pot is moved, and a skewer passes through it with no resistance.
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4
Remove the pot from the heat and allow the pork to cool entirely in its braising liquid. The pork reabsorbs the seasoned broth as it cools; removing it while hot will cause it to dry. The surface of the liquid will set lightly as the collagen gels — a reliable sign the braise has succeeded.
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5
Lift the cooled pork from the liquid and set aside. Pour the braising liquid into a wide pan and reduce over high heat until halved in volume and visibly syrupy. Return the pork cubes to the pan and glaze for 10 minutes over high heat, turning the cubes so each face receives the lacquer. The surface of each cube should be deep mahogany and glossy.
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6
Serve immediately, with karashi mustard alongside. The mustard cuts the richness of the rendered fat.