To prepare the octopus
Heat the grapeseed oil in a 6-quart pot over medium-high heat. Add the onion, carrot, celery, garlic, and thyme and sauté until the onion is translucent, about 5minutes. Add the water and wine and bring to a simmer. Add the octopuses and bring the liquid to a near simmer, then reduce the heat to low, cover the octopuses with a small weight so that they are submerged, and simmer very gently for about 2hours, or until a small knife glides easily through one of the octopus arms. (Never allow the water to boil or the skin will fall away; slow and gentle is the key.)
Remove the octopuses from the cooking liquid and let cool. (Discard the cooking liquid.) Cut each of the 8 octopus legs away from the head; discard the head.
To marinate the octopus
Mix all the marinade ingredients in a large bowl to blend. Add the octopus legs and toss them to coat completely. Refrigerate for 6 hours.
Meanwhile, to make the sauce
Mix the yogurts in a small bowl to blend. Season to taste with salt. Cover and refrigerate.
To prepare the garnishes
Cut the grapefruit segments into small dice; set aside.Remove the stems from the cauliflower florets. Using a mandoline, cut the florets into 2-inch-thick slices.
To sear the octopus
Heat the olive oil in a large cast-iron skillet over high heat. Remove the octopus legs from the marinade and lightly shake off the excess marinade; the octopus should still be well coated. Add the octopus legs to the hot skillet and cook for about 3 minutes per side, or until almost black. Transfer to a cutting board.
To serve
Spoon about 1 tablespoon of the yogurt sauce into the center of each of eight plates. Mix the grapefruit and cauliflower in a bowl and season to taste with salt and white pepper. Place about 1 tablespoon of the grapefruit mixture on top of the yogurt on each plate. Cut each octopus leg into 3 pieces and arrange them on top of the grapefruit mixture. Lightly drizzle with olive oil, sprinkle lightly with fleur de sel, and serve immediately.