Blanch the octopus by bringing 6 quarts of water, salt and rice wine vinegar to boil. Dangle the octopus by the head and swirl the legs in the blanching liquid for 30 seconds; then place the entire octopus in the liquid and simmer for 10 minutes. Take the octopus out and shock in ice water. When cool, cut the legs from the body and reserve.
Braise the octopus: Sweat the onion in the olive oil until tender. Then, deglaze the pan with the sake. Add the remaining ingredients, 1 quart water, the octopus legs, and bring to a slow simmer. Cover and continue to simmer for 1 1/2 hours or until octopus legs are tender.
For the black pepper condiment: Heat the oil in a large pot. Add the ginger and garlic and cook until golden. Then add the scallions and cook over medium heat until soft. Add the black peppercorns and cook until very fragrant. Add remaining ingredients and bring to a boil. Simmer for 8 minutes or until the sauce coats the back of a spoon. Remove from heat and puree until coarsely ground.
For the tarragon puree: Combine the shallots, extra virgin olive oil, and salt in a pot and cook on medium heat, covered, until shallots are completely soft but have no color. Drain the oil. Cool in a bowl over ice, then put in blender with tarragon and puree until completely smooth. Store in a squeeze bottle.
Prepare the onion salad by putting the onions and ají dulce in a small bowl and seasoning with olive oil, lime segments, salt and pepper.
Heat a grill over high heat and oil the grill grates. Toss octopus legs with olive oil and salt and char on grill until hot throughout. Place the hot octopus legs in a bowl and toss with Black Pepper Condiment until coated. Place a line of the onion salad on the left side of a plate and top with the octopus legs in a row. Put a dollop of tarragon puree next to the octopus and swipe with a spoon. Garnish octopus with tarragon leaves and serve.