1. Make the fish stock first. Heat the oil in a large saucepan over medium heat and fry the onion and celery until softened (about 5 minutes). Increase heat to high,add the fish heads,bones,prawn heads and shells,garlic,lime leaves,tomato,lemongrass and chilli,stir thoroughly. Add the wine and reduce until it has nearly all evaporated (4-5 minutes). Stir in the tomato paste and water,bring to a low boil,then reduce heat to medium-low and simmer gently for 30 minutes.
2. Strain the stock through a fine sieve. Reserve about 1 cup to cook the cous cous and 1½ cups for cooking the seafood. Freeze the remainder.
3. Place the mussels and clams in a pot with a tight-fitting lid with about ½ cup of fish stock,cook over high heat,covered,until they open (2-3 minutes),remove with a slotted spoon,add another cup of stock and reduce by half (about 8-10 minutes). Reduce heat to medium,add the blue-eye and prawns and cook gently until just cooked through (2-3 minutes). Return the mussels and clams to the pot,fold through and check the seasoning:the juice from the molluscs should have made it salty enough. If not,add a little salt.
4. To prepare the cous cous,heat the amount of stock that the packet prescribes for a cup of cous cous (usually 1 cup),add everything to a bowl except the coriander,pour the heated stock over the top and allow it to soak in (about 10 minutes). Once the stock is absorbed,use a fork to fluff up the cous cous,add the coriander and fold through.
5. Spoon the couscous on the side of 4 bowls,add seafood and sauce to each bowl,scatter with mint and give a good grind of pepper. Serve with some harissa on the side,if desired.