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1
If the octopus has not been frozen for 48 hours, submerge it in a large pot of plain unsalted water and bring to a gentle boil. Cook for 45–60 minutes. The dish is done at this stage when a thin-bladed knife passes through the thickest section of a tentacle without resistance.
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2
Lift the octopus from the water and drain thoroughly. Pat the surface as dry as possible before it enters the roasting vessel — residual water fights the oil bath and prevents crisping.
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3
Heat the oven to 220 °C. Arrange the octopus tentacles in a single layer in a cast-iron or earthenware roasting dish. Scatter the smashed garlic and bay leaves across the pan, then pour the olive oil over and around until the tentacles are generously bathed — not merely filmed. This is where the dish lives or dies: too little oil and the lagareiro technique fails; the tentacles will tighten and dry rather than roast silky.
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4
Roast at 220 °C for 20–25 minutes. The dish is done when the skin at the tips of the tentacles blisters and the very ends begin to colour to a deep caramel bronze, while the thicker sections remain yielding. The oil in the pan will be fragrant with octopus juices, garlic, and bay.
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5
If roasting potatoes in the same dish, add the halved waxy potatoes to the olive oil bath before the octopus goes in, or transfer them to a separate tray and ladle some of the infused cooking oil over them at the halfway point. They are done when golden on the cut face and tender through to the centre when pressed.
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6
Serve the octopus in its cooking oil, with the potatoes alongside. Spoon the garlic-and-bay-infused oil from the pan over the whole dish at the table.