Pre-heat oven to 300F.


In a large Dutch oven, heat 2 tablespoons of olive oil over high heat until almost smoking. Add the garlic and 1 teaspoon of the red pepper flakes and cook over high heat for 2 minutes. Add the octopus to the pan and cook on all sides over high heat until it has changed color and has released its liquid. Add the wine cork, cover the pan with aluminum foil, and bake for 1½ to 2 hours or until the octopus is quite soft. Cool to room temperature.


Meanwhile, bring 3 quarts of water to a boil and add 1 tablespoon of salt. Fill a large bowl three-quarters of the way full with ice water and keep it close by. Cook the potatoes and sliced onions in the boiling water for 3 minutes, until somewhat tender but still firm. Drain the vegetables and plunge them into the ice bath. Drain again and spread on paper towels to dry.


In a small, heavy-bottomed saucepan, bring the tangerine juice and remaining teaspoon of red pepper flakes to a boil and cook until reduced by half, about 8 minutes. Allow to cool, then place in a small bowl with the champagne vinegar. Slowly whisk in the ½ cup of olive oil to barely emulsify and season with salt and pepper.


Carefully separate the octopus tentacles from the head. Cut the tentacles into 3-inch pieces; cut the head in half.


Place the potatoes, sliced onion, and chopped red onion in a large bowl and toss with the red wine vinegar, the remaining 3 tablespoons of olive oil, and salt and pepper. Divide the mixture evenly among four chilled dinner plates, mounding the potatoes on the center of each plate. Top with some of the octopus tentacles, drizzle with tangerine citronette, garnish with chives, and serve.