How to Make It
First, you need to tenderise the octopus. Fishmongers can do this for you by spinning the octopus in an oyster-cleaning machine. If you plan to tenderise it yourself, freeze the octopus for 2–3 days, then defrost it completely and beat the octopus all over with a mallet or a rolling pin to break down the muscles.
Cook the potatoes in their skins in boiling salted water until soft. When they are cool enough to handle, peel them and cut into 1 centimeter (1/2 inch) thick slices.
Bring 2 liters (68 fluid ounces/8 cups) water, the bay leaves and peppercorns to the boil (do not season the water with salt). Put the octopus in the boiling water and cook for 12 minutes. Strain the octopus and let it cool for 5 minutes.
Remove the octopus skin and cut the tentacles into 5 millimeters (1/4 inch) slices. Transfer to a serving dish. While the octopus is still warm, add the olive oil, vinegar, potato, onion, tomato, cucumber, garlic, parsley and season with salt and pepper to taste. Mix well and serve.