Put the octopus into a large pot of water and bring to a boil. (Add a wine cork to the water to ensure tenderness) Once it reaches a boil, lower the temperature to medium-low and cover the pot. Leave it for about 1 ½ hours until tender.
When the octopus is soft, the tentacles with remain a bit firm, peel the skin by rinsing the octopus under warm water. Chop the octopus into pieces of about 1 ½ inches in length.
Peel the potatoes and boil them until fork tender, drain the water and slice the potatoes.
In a bowl, combine the octopus, potatoes, onion, garlic and tomato sprinkle some salt and pepper to taste, then drizzle a generous amount of olive oil and red wine vinegar and the juice of ½ lemon. Mix well and garnish with the fresh parsley, lemon slices and serve.