Step 1Blend the garlic, shallots, chiles, ginger, turmeric, lemon grass and 2 tablespoons of oil in a food processor.
Step 2Blanch the fish heads in boiling water for about 3 minutes, then soak them in ice water for a few minutes. Drain and set aside.
Step 3Heat the remaining 2 tablespoons of oil in a large pan over low heat. Cook the garlic mixture with the curry leaves, stirring frequently until fragrant, about 5 minutes. Add the curry powder and continue stirring for 1 minute. Add 1 cup of water and simmer for about 3 minutes. Add the remaining 1 cup of water, tamarind juice, coconut milk, fish heads, okra and eggplant. Bring to a boil over medium heat and simmer, uncovered, until the fish is cooked, about 5 minutes. Season with the salt and sugar. Garnish with the green onions and tomatoes.