Preheat oven to 170°C. Prepare your octopus. Cut the head off beneath the eyes and then cut it in half lengthways, removing the eyes and the inside of the head. Using a sharp knife, or scissors, separate the tentacles from each other, and discard the shell-like beak (the mouth) and the mouth area. Place the octopus along with all the other ingredients into a large ovenproof pot and bring to the boil. Alternatively, bring the liquids to the boil and pour over the octopus in a ceramic or stainless steel deep-sided roasting dish. The octopus should be covered with liquid; if it isn’t, top it up with extra water. Place a sheet of baking parchment on top of the liquid, then seal the dish really tightly with aluminium foil or a lid and bake in the centre of the oven for 1¾ hours. Take it out and leave to cool. At this point it’s ready to be eaten, but it will also be delicious served cold.
For the coriander dressing, using a stick blender or bar blender, blitz everything except the extra virgin olive oil and the lemon juice together until smooth. Stir in the olive oil and then the lemon juice and taste for seasoning.