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.