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1
Place the pork belly blocks in a large pot with the negi and ginger. Cover with cold water. Bring to a gentle simmer and cook uncovered for 30–40 minutes (shita-yude). The objective is the elimination of kusami — the surface of the water should carry a grey foam and rendered fat; the pork will have lost its raw smell entirely. Drain, discard the aromatics, and rinse each block under cold water.
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2
Transfer the pork blocks to a clean, wide pot in a single layer. Pour over the dashi, sake, mirin, and half the soy sauce (50 ml). Set the heat to the lowest possible level — the target is a bare 80–85°C, never a rolling boil. Place an otoshibuta (drop lid) directly on the surface of the liquid, covering the pork completely. Simmer for two to three hours. This is where the dish lives or dies: too high a heat will toughen the meat and render the fat grainy; the correct heat produces a fat layer that is fully translucent and quivering, with meat that is tender but retains its form. Test by lifting a block: it should wobble like set gelatin — firm enough to hold, on the verge of collapse.
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3
Add the remaining 50 ml of soy sauce to the braising liquid. Remove the otoshibuta. Raise the heat to a moderate simmer and reduce the liquid, turning the pork blocks gently and basting frequently, until the sauce concentrates to a glossy coat that clings to each surface — this is tsuya-dashi. The glaze is ready when it is translucent and savoury, not thick and sticky.
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4
Optional but recommended: after step 2, remove the pot from heat, allow to cool, and refrigerate overnight. Lift away the solidified fat cap from the surface before reheating. Proceed to step 3. The flavour will be more concentrated; the richness more controlled.
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5
Plate the blocks with a small quantity of reduced braising liquid spooned over. Serve karashi mustard alongside. The cut surface of each block should show distinct, fully translucent fat above compact, yielding meat.