If your octopus is not frozen it is best to freeze it first as it helps to tenderize it. Then the day before you want to prepare this, place the frozen octopus in a large bowl in the refrigerator to defrost. It will release a lot of water.
Fill a large pot with water, add the whole peeled onion and bring to a boil. Do not add any salt at this time.
When the water is boiling take the octopus by the head and gently dip the legs into and out of the water about 3 times.
This allows the skin to stay in tact during the cooking process.
Now put the whole octopus into the pot and boil for about 45-50 minutes.
Meanwhile you can peel the potatoes and cut them into slices.
The octopus is done when it is fork tender, remove it from the boiling water and set aside to cool.
Add the potatoes into the boiling water and cook about 10 minutes until tender.
While the potatoes are cooking you can slice up the octopus into ½ inch size pieces.
Arrange the potatoes on a platter and the octopus on top of the potatoes.
Add some salt, sprinkle paprika to taste and douse with some olive oil.