Place the octopus in a deep saucepan with the white vinegar, peppercorns, celery onion, and water.
There should be enough water to cover the octopus.
Add 2 or 3 wine corks.
Bring to a simmer and cook for 1 hour.
Drain the octopus and transfer it to a bowl of ice water.
Allow it to cool completely.
Slice the octopus in 1-inch (2.5 cm) pieces. Mix the olive oil, red wine vinegar, salt, pepper, and parsley together and toss with the octopus. Allow to marinate at least 1 hour.
Serve the octopus with marinated radicchio on the side.