Put the fish in a narrow (non-metallic) mixing bowl. Mix the tomatoes, onion, coriander, garlic, chili pepper, lime juice, oil and salt in a blender. Mix to a smooth mixture and pour over the fish. Allow to marinate for 30 minutes to 1 hour.
For the casserole:
Heat the olive oil in a large pan on medium heat. Add the diced sweet potato and fry for 10 to 15 minutes until soft. Add some water if necessary, so that the potatoes do not stick to the pan.
Add the zucchini, the green and red pepper and let it cook until everything is soft. This will take 5 to 7 min. Add some more water in the pan, if necessary.
Add the water or fish stock and then incorporate the coconut milk. Bring to a boil, lower the heat and let it simmer for 2 to 3 minutes.
Add the fish and the marinade and let it simmer for 10 to 15 minutes or until the fish easily disintegrates with a fork.
Stir in the spring onion and coriander just before serving. Serve alone or with rice.