Mix the lemon juice with the vinegar, cumin seeds, curry powder and salt in a shallow non-metallic bowl. Add the fish and turn to coat evenly. Cover and marinate in the refrigerator for 25–30 minutes.
Meanwhile, heat a wok or large frying pan with a lid over a high heat and add the oil. When the oil is hot, add the onion, garlic and ginger. Reduce the heat and cook gently for 10 minutes, stirring occasionally.
Add the tomatoes and agave syrup, stir well and bring to the boil. Reduce the heat, cover and cook gently for 15–20 minutes, stirring occasionally.
Add the fish and its marinade, and stir gently to mix. Cover and simmer gently for 15–20 minutes until the fish is cooked through. Ladle into shallow bowls and serve with steamed basmati rice.