Ask your fishmonger to clean and prepare the octopus (take the nasty bits out) as it's not exactly a pleasant job. Even if they do it for you, you should clean it again at home (if you have children, ask them to do it, especially if they're boys - they'll love it!)
You need a big pan three quarters full of boiling water.
Add one peeled onion, a few bay leaves, a few grinds of black pepper and 2 medium size peeled potatoes.
Scorch the octopus by holding it with tongs and dipping it in to the boiling water 3 or 4 times.
When you've done this immerse it in the water fully. Cover with a lid and leave to simmer for approx 45 mins to 1 hour. (It may need longer cooking time for a large octopus). You should be able to put a knife through easily, but without it being too soft.
When it's done take out and slice up as in picture, arrange on a plate and drizzle with olive oil. Sprinkle with a little sweet smoked paprika (only a tiny amount if children eating it too as it's quite strong). If you can get over it being octopus (which lots of people I know can't) it's really delicious. Edit: I meant to say to slice the potato up too and arrange on the plate first.