1. Wash fish thoroughly under cold, running water. Pat dry using paper towels. Place in an ovenproof dish. Pour orange juice over.
2. Sprinkle with mustard, salt, pepper, ground coriander, cardamom seeds, 2 cloves garlic, crushed, and 15 ml (1 tbsp) ginger. Cover with foil and bake at 180 ºC for 20 to 25 minutes, or until fish flakes with a fork. Do not overcook fish, rather undercook it as it will firm as it cools. Remove and cool to room temperature. Chill for about 15 minutes. Drain off 125 ml (1/2 cup) orange juice and reserve.
3. Sauté tomatoes, sugar, chilli, remaining garlic, crushed, and remaining ginger in a frying pan. Cook over high heat for 5 to 8 minutes. Cool to room temperature. Purée mango and coriander, add tomato mixture and reserved orange cooking liquid. Purée until smooth.
4. Spoon over fish. Marinate, covered, in the fridge for an hour.
5. Sprinkle fresh coriander over fish. Arrange on a platter and pour any pan juices around fish. The fish is very tasty served slightly warmed or at room temperature. Serve with mushroom rice (see recipe).