How to Make It
Heat grapeseed oil in a 6-quart enameled cast-iron casserole over medium-high. Add onion, carrot, celery, garlic, and thyme; cook until onion is translucent, about 5 minutes. Add 8 cups water and wine, and bring to a simmer. Add octopus; bring liquid to a near simmer, and reduce heat to low. If necessary, cover octopus with a plate so that it remains submerged. Simmer until a small knife glides easily through octopus arm, about 2 hours. (Do not allow water to boil or the skin will fall away; slow and gentle is the key.)
Remove octopus from cooking liquid, and let cool; discard cooking liquid. Cut octopus legs away from head; discard head.
Combine tandoori paste, tomato sauce, honey, molasses, and sherry vinegar in a large bowl. Add octopus legs; toss to coat. Cover and refrigerate 6 hours or up to overnight.
Combine goat yogurt and Greek yogurt in a small bowl. Season to taste with salt. Cover and refrigerate until ready to serve.
Heat olive oil in a large cast-iron grill or skillet over high. Remove octopus legs from marinade, and lightly shake off excess marinade (The octopus should still be well coated). Add octopus legs to skillet; cook until almost black, about 3 minutes per side. Transfer to a cutting board, and cut each octopus leg into 3 pieces.
Spoon about 1 tablespoon yogurt sauce into center of each of eight plates. Combine grapefruit and cauliflower in a bowl, and season to taste with salt and white pepper. Place about 1 tablespoon grapefruit mixture on top of yogurt sauce on each plate. Arrange 3 octopus leg pieces on each plate. Lightly drizzle with olive oil, sprinkle lightly with fleur de sel, and serve immediately.