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1
Rub the cleaned octopus thoroughly with sea salt, working it into the skin. Rinse under cold water. This firms the flesh and begins the tenderising process. The skin should feel slightly tacky to the touch.
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2
Place the octopus in a pot of cold, lightly salted water. Bring slowly to a gentle simmer. Do not boil hard; a rolling boil toughens the flesh. The water should show a steady, low simmer — small bubbles rising at the edges, the surface barely disturbed.
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3
This is where the dish lives or dies. Begin testing the thickest tentacle with a chopstick after 30 minutes. Drive the chopstick through the flesh: firm resistance means continue; slight resistance — the drag of a perfectly cooked potato — means remove from heat immediately; no resistance at all means the window has passed. There are no second chances. Pull the octopus the moment slight resistance is felt.
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4
Transfer the octopus to a tray and allow to cool to room temperature. Do not plunge into ice water; allow the residual heat to finish the flesh gently. The tentacles will tighten slightly and the skin will settle to a deep, uniform purple-brown.
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5
Slice the cooled octopus into bite-sized pieces. The cut surface should show a clean, slightly translucent interior with no visible toughness or stringiness. Dress with shoyu, sesame oil, sliced spring onions, and toasted sesame seeds. Toss to coat evenly. Taste for salt before serving.