Preheat the oven to 350 degrees F. Pat dry the fish fillets, season with salt and pepper, and arrange in a shallow baking dish. In a blender or food processor combine the onion, garlic, gingerroot, tamarind paste, sesame oil, cumin, salt to taste, and 2 tablespoons water and blend until smooth. Pour mixture over fish, coating both sides. Bake fish, covered with foil, in the oven for 20 to 25 minutes, or until it flakes easily with a fork. Transfer fillets to a serving dish, correct seasoning, adding more salt if necessary, and drizzle with lemon juice. Garnish with mint.