Place the octopus in a large pot together with all the remaining ingredients for boiling. Add enough water to just cover the ingredients. Cover the pot with a lid and bring to a boil.
Reduce the heat to medium and cook for approximately 1 hour, depending on the size of the octopus. The aim is for the octopus to be tender but not falling apart. If you overcook it, it may become spongy and lose its flavour.
To check if it is ready, cut a small piece from one of the tentacles with a knife. If it cuts easily and does not feel rubbery, it is done.
While the octopus is cooking, prepare the dressing by mixing all the ingredients together.
Drain the octopus and cut the tentacles into pieces. Cut the head in half so that you have two halves. Remove the outer skin by gently scraping the octopus with a knife. Alternatively, you can remove it by rubbing the tentacles with a paper towel. The aim is to remove the thin fatty layer, not the suckers.
While the octopus is still warm, place it in a resealable zip-lock bag. Pour over half of the marinade. Remove as much air as possible from the bag and seal it. Place the bag inside a container, in case of any leakage, and refrigerate for 24–48 hours. Keep the remaining half of the marinade in a covered container in the refrigerator.
Remove the octopus from the refrigerator at least 1 hour before cooking so that it can come to room temperature.
Take the pieces of octopus out of the bag and transfer the marinade from the bag to a bowl. Cook the octopus over a very hot grill or in a hot pan, continuously brushing it with the marinade, until beautifully browned and caramelised with a lovely crust.
Transfer the octopus to a serving platter and drizzle over any remaining marinade. Serve hot or at room temperature.