Chop the boiled octopus into thin slices, anywhere from between 1 to 4mm. The thinner, the better.
Arrange it neatly on a plate, wrap it in plastic, and place it in the refrigerator for later use.
Mince the onion finely and soak it in water for about 5 to 10 minutes to get rid of the bitter taste and strong smell. Then, cut the cucumber and tomato into small pieces. Combine all vegetables in a bowl and add the above-mentioned seasoning. Mix until well combined.
Remove the octopus plate from the fridge and add the prepared vegetable dressing on top. Serve immediately and enjoy!