Finely mince together the coriander and the garlic. Put in a bowl with the spices. Coat the fish with the spice mixture and set aside.
Pour the oil and water into a large casserole. Place over the heat and bring to a gentle boil before carefully lowering in the fish. Cook over a low heat for around 15 minutes.
Add the tomato and the preserved lemon to the pan. When the sauce is thick, remove the pan from the heat and carefully lift out the fish.
Serve with the sauce spooned on top.