Place the cut up octopus in a large cast iron pot. Add the olive oil, onion, garlic, lemongrass and salt and white pepper to taste. Turn the heat up to high and cover tightly with a lid.
Cook for 15 minutes, then lower the heat to medium-high, without uncovering, and cook for another 20 minutes. Remove the lid and make sure it’s cooked and tender. If not, cook a bit more.
Meanwhile, in a microwave-safe dish, place the two whole potatoes and parsnip and cover with two slices of bacon on top.
Cover with plastic wrap and microwave for 25 minutes. The potatoes and parsnip will cook and absorb the smokiness from the bacon.
Discard the bacon. Peel potatoes and parsnips and smash with a fork, adding the arbequina olive oil in thin streams.
Spoon the mash onto 4 plates and cover with equal parts of the octopus and its juices.
Sprinkle octopus with panko and Maldon salt.