Step 1Heat oil in large skillet over medium-high heat. Add onion, curry powder, cumin, tamarind paste, jalapeno, garlic, salt and sugar. Saute until onion is translucent, about 3 minutes.
Step 2While onion cooks, dredge shrimp, scallops and fish in flour, shaking off excess. Add to onion. Stir-fry over high heat, about 3 minutes, then add vermouth. Cook, stirring, until fish is cooked through and no longer pink. Remove from skillet and place on baking sheet lined with paper towels to drain. Refrigerate to speed cooling.
Step 3Puree tomatoes with tomato juice in food processor until thick. Divide among 4 (16-ounce) bowls, layering with cucumber in attractive presentation. Place equal amounts of seafood, fish and onion mixture in each bowl. Top each with one-fourth of crab. Place dollop of sour cream, about 1 tablespoon, over top of crab. Garnish with green onions and mint. Serve immediately.