Prepare the Octopus: In a large pot, bring salted water to a boil. Add the octopus, reduce the heat to low, and simmer until tender, about 45-60 minutes. Once cooked, remove from the water and let cool. Then, cut the octopus into bite-sized pieces.
Fennel and Onion Mixture: While the octopus is cooking, prepare the fennel by slicing it thinly. Soak the thin slices of red onion in a bowl of cold water for 10 minutes to mellow their flavor, then drain.
Orange Supremes: Cut the top and bottom off the oranges. Stand them on one end and carefully slice away the peel and white pith. Over a bowl to catch the juices, cut between the membranes to release the orange supremes. Squeeze the remaining membrane to extract any additional juice.
Dressing: Whisk together olive oil, red wine vinegar, honey, and the juice collected from the oranges. Season with salt and pepper to taste.
Combining the Salad: In a large mixing bowl, gently toss the octopus, fennel, drained onion, and orange supremes. Drizzle with the dressing and toss again to coat evenly.
Final Touches: Sprinkle with chopped parsley and give the salad one final toss. Adjust seasoning with salt and pepper if needed.
Chill and Serve: Let the salad chill in the refrigerator for at least 15 minutes before serving to allow the flavors to meld.