Rinse the fish with cold water, pat it dry with kitchen paper, and remove any grey membrane. Cut the flesh from the cartilage backbone in two long pieces. Cut each piece into 8-12 equal-size chunks.
Peel the prawns, removing the legs and heads but leaving the tails on if possible. Run a sharp knife along the back of each prawn and pull away the dark digestive thread. Rinse and pat dry with kitchen paper.
Mix the garlic, ginger and chilli with the oil in a large bowl. Add the monkfish and the prawns and mix well. Cover with cling film and chill for 1 hour. If you are using wooden skewers, soak them in cold water for at least 30 minutes before using.
Meanwhile, make the salsa. Put a kettle on to boil. Cover the tomatoes with boiling water and leave for 1 minute. Then skin and deseed them and chop the flesh into a chunky purée. This must be done by hand, not in a food processor.
Dry-fry the coriander seeds in a heavy-based frying pan over a high heat for a few seconds until they release their aroma. Then crush them in a spice mill or with a pestle and mortar and add them to the tomato. Mix in the chilli, garlic, lemongrass and lime juice and season well. Cover and chill for 30 minutes or more.
When you are ready to cook, heat the grill to medium-high. Thread the monkfish and prawns onto four skewers and grill for 3-4 minutes on each side until the prawns are firm and pink but not overcooked. Season the kebabs to taste, garnish with the lime wedges and serve with the salsa on the side.
Variation: cook the kebabs on a barbecue rather than under the grill for a smokier flavour. Or cook them in a ridged, iron grill pan over a medium-high heat.