1.Shell and devein 8 prawns, keeping tails intact; place shells and heads in a large bowl. Reserve shelled prawns in a small bowl; cover, refrigerate, until required. Add remaining unshelled prawns to large bowl.
2.Heat half the oil in a large saucepan over high heat; cook onion, garlic, carrot and celery, stirring, for 5 minutes or until onion begins to soften. Add prawn mixture; cook, stirring 5 minutes or until prawns change colour. Stir in herbs and paste. Add the water and wine; bring to the boil. Reduce heat to medium-low, add fish; simmer 20 minutes, skimming the surface occasionally with a ladle to remove scum.
3.Remove fish from stock; cover until required.
4.Increase stock to medium heat, add tomatoes and potatoes; season. Cook for 40 minutes or until soup has reduced slightly. Cool 10 minutes.
5.Blend soup, in batches, until smooth. Push blended soup through a fine sieve into a clean medium saucepan over heat; return to the boil.
6.Meanwhile, heat remaining oil in a large frying pan over medium-high heat; cook reserved prawns for 1 minute each side or until just cooked through.
7.Stir crème fraîche and juice into soup; season to taste. Ladle soup into bowls; top with flaked fish, prawns and extra chopped parsley.