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1
Combine the dashi, soy sauce, sake, and konbu in a donabe or heavy-based pot. Bring to a gentle simmer over medium heat. The broth should smell clean, deeply saline, and of the sea — not sharp or raw.
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2
Add all the negi pieces to the simmering broth. Cook over a steady, gentle simmer until the negi have softened completely — they should offer no resistance when pressed and begin to approach translucence at the edges, their sharpness converted entirely to sweetness. This takes time and cannot be rushed. Do not add the fish until this stage is complete.
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3
Add the fatty tuna pieces to the broth in a single layer and simmer for 2–3 minutes only. This is where the dish lives or dies: the tuna must be removed from the heat while its centre remains faintly pink. The fat will visibly disperse into the broth, enriching and clouding it. Grey flesh throughout is overcooked; the dish is destroyed.
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4
Taste the broth immediately before serving. The soy presence should be robust and forward in the Edo manner — not a background note. Adjust with soy if needed. Serve directly from the pot at the table.