Put the onion, cloves and 1 tablespoon salt in a large saucepan and add the measurement water. Bring to the boil. Using tongs, dip the octopus in and out of the water about 4 times, returning the water to the boil before re-dipping, then immerse the octopus completely in the water. (This helps to make the flesh tender.) If there are several pieces of octopus, dip them 1 at a time.
Reduce the heat and cook the octopus very gently for 1 hour, then check to see whether it's tender. Cook for a further 15–30 minutes if necessary. Leave it to cool in the liquid, then drain, cut into bite-sized pieces and place a non-metallic bowl.
Mix the oil with the garlic, parsley, vinegar and salt and pepper to taste and add to the bowl. Mix well, cover and chill for several hours or overnight. Serve the octopus with bread for mopping up the juices.