In a large enough pot, boil the whole octopus in a slightly salted water. Cook it for about an hour, or until meat is soft enough to be easily pierced with a fork.
Once the octopus is cooked, drain the water and let the octopus meat cool down. Once it's safe to touch, cut the octopus into small pieces. Put in a bowl and set aside.
Chop the onions, peppers, and parsley. If you want to add other vegetables, slice them into small cubes, too.
In the bowl with the sliced octopus, add the lemon, extra virgin olive oil, salt, and pepper. Make sure to mix all the ingredients well.
Next, add the chopped onions, peppers, parsley, and other fresh vegetables you want to add. Toss all the ingredients in the bowl.
Let it sit in the refrigerator for a few hours before serving. It's important to let all the flavors blend before serving.