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1
Place the pork belly cubes in a large pot and cover with cold water. Bring to a vigorous boil over high heat and cook for 5 minutes. Drain, discard the water, and rinse each cube under cold running water, removing any coagulated protein or blood. The cubes should be visibly clean and pale — this is the condition required before further cooking.
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2
Return the blanched pork to the pot. Cover again with cold water and add the sliced ginger and whole spring onions. Bring to a gentle simmer and cook, uncovered, for 30 minutes. The water should carry a faint savory steam and the pork should show no trace of the harsh raw odor that characterized it before blanching.
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3
Lift the pork cubes from the aromatic liquid and transfer to a wide, heavy braising pot in a single layer. Discard the ginger-and-spring-onion water. Add the dashi, sake, soy sauce, mirin, and sugar. The cubes should be largely — though not fully — submerged. Place an otoshibuta (drop lid) directly on the surface of the liquid, fitting snugly against the tops of the cubes. Bring to a gentle simmer.
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4
This is where the dish lives or dies. Maintain the gentlest possible simmer — the surface should shiver, not roll — for a minimum of 2 hours and up to 3, turning each cube once at the halfway point. At no stage should the heat be sufficient to agitate the cubes violently; aggressive boiling will cause them to collapse and shed their skin. At the end of the braise, press a cube with a fingertip through a folded cloth: it must yield with almost no resistance, trembling under the slightest pressure, while still holding its shape externally. If any resistance remains, continue braising in 20-minute increments until this test is passed.
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5
Remove the pot from the heat. Allow the cubes to cool in the braising liquid, then refrigerate the entire pot, uncovered, overnight. By morning the liquid will have set to a loose jelly around each cube and a solid cap of rendered fat will have formed on the surface. Lift and discard this fat layer — the sauce beneath should be clear, deep amber, and trembling.
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6
Return the pot to a low flame and reheat gently until the jelly dissolves back to liquid and the cubes are heated through. Ladle into deep bowls, spooning the reduced braising liquid over each cube. Serve with karashi mustard alongside. The finished surface of each cube should be glossy and lacquered; the skin should move perceptibly when the bowl is set down.