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1
Place the pork belly pieces in a heavy pot and cover with cold water. Add the leek and ginger. Bring to a gentle boil, then reduce to a slow simmer and hold for 30–45 minutes — this is the shitani. Lift the pork out and discard the liquid, aromatics, and accumulated scum. Rinse each piece under cold water. The surface of the pork should be clean, pale, and free of grey foam.
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2
Return the pork to the rinsed pot, fat cap facing down. Add the soy sauce, sake, mirin, sugar, and water. Bring to a bare simmer — bubbles should break the surface slowly, one or two at a time, never a rolling boil. Cover with a drop lid (otoshibuta) or cartouche and braise on the lowest possible heat for 2–3 hours. This is where the dish lives or dies: high heat will tighten the muscle fibres irrevocably and prevent the collagen from melting; patience at a near-still simmer is the technique. Test at two hours by inserting a chopstick through the fat cap — it should pass with only slight resistance and exit cleanly. If it grips, continue for a further 30 minutes and test again. The fat, when ready, is translucent and trembling.
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3
Transfer the braised pork pieces to a tray or shallow dish and set aside. Return the braising liquid to a moderate heat, uncovered, and reduce to approximately half its volume, stirring occasionally. The liquid should thicken to a glossy, lacquered consistency that coats a spoon in a deep mahogany film.
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4
Return the pork to the reduced liquid and turn each piece to glaze on all sides over low heat for 3–5 minutes. Do not discard the braising liquid before this step; it is the flavour-concentration stage. The finished pieces should be lacquered and deeply coloured, the reduced glaze clinging without pooling.
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5
Serve each portion with a small mound of karashi alongside. The sharpness of the mustard should cut directly against the fat richness — taste a piece first with mustard and then without. The contrast between trembling unctuous fat and the mustard's heat is the flavour identity of the dish.