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1
Clean the octopus if the fishmonger has not done so, removing the beak and ink sac. Rinse well under cold running water. Do not dry it: the surface moisture is part of the cooking liquid. Place the whole octopus in a heavy terracotta or cast-iron pot of a size that holds it snugly.
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2
Add the San Marzano tomatoes, garlic cloves, olive oil, capers, Gaeta olives, and peperoncino directly to the pot. Nothing more — no water, no wine, no stock. The octopus will release abundant liquid of its own; any addition dilutes the flavour of extraordinary depth that is the point of the dish.
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3
Mix the flour and cold water to a stiff, pliable paste. Press this paste firmly into the gap between the lid and the rim of the pot, sealing the vessel completely so that no steam can escape. This is where the dish lives or dies: a broken seal means lost liquid, lost pressure, and an octopus that will never reach true tenderness in its own braising medium.
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4
Set the pot over the lowest possible heat. The crust must not colour; the pot must barely murmur. Cook undisturbed for a minimum of 45 minutes and up to 1 hour for an octopus of 1–1.5 kg. Do not lift the lid, do not break the seal, do not adjust the heat upward. The cook's only cue during this period is the faint, contained hiss of steam pressing against the flour crust — audible proof that the seal is holding.
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5
Break the flour crust at the table or in the kitchen. The octopus is done when a fork meets no resistance anywhere along the thickest part of the mantle, and the braising liquid has concentrated into a rust-red sauce pooling around the tentacles.
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6
Lift the octopus from the pot and cut the tentacles with scissors. Reserve the sauce. If serving in the Neapolitan sequence, toss the hot sauce with cooked, well-drained spaghetti and serve as the primo; bring the tentacles in their remaining sauce to the table as the secondo. The sauce is ready when it coats the spaghetti in a glossy film without pooling on the plate.