Heat the oil in a large frying pan, add the onion and fry gently for about 10 minutes until golden brown. Add the chilli, garlic, ginger and dried spices and fry for a further minute until fragrant.
Add the tomatoes and coconut milk to the pan. Bring to the boil, then reduce the heat and simmer for about 10 minutes until the curry sauce has thickened. Add the monkfish and prawns to the pan and cook for 3–4 minutes. Finally, add the mussels (first discarding any that don't shut when tapped) and cook for a further minute or so until they have opened, discarding any that don't.
Season and stir through the chopped herbs. Serve with basmati rice.