Put all the fish in a large saucepan or or casserole. Add 225ml (8fl oz) water, the carrot, shallots, celery, radishes, bouquet garni and salt. Slowly bring to the boil, then reduce the heat, cover and poach gently for 10 minutes. Remove from the heat and leave to cool in the cooking liquid.
When completely cool, discard the bouquet garni. Remove the fish from the pan and place on a tray. Slice the monkfish into medallions and break the remaining fish into generous bite-sized pieces. Place all the fish in a large mixing bowl. Strain the cooking liquid into a jug and transfer the strained vegetables to a plate. Add the parsley and some pepper. Set aside.
Place 125ml (4fl oz) of the reserved cooking liquid in a heatproof bowl. Add the lemon juice, paprika, saffron and nutmeg and stir to mix. Add salt to taste. Sprinkle the gelatine over the surface and leave to soak for 5 minutes.
Heat a large frying pan of water to just below boiling point. Set the bowl of gelatine bouillon in the pan and heat gently for 3-5 minutes until the gelatine has dissolved completely and the bouillon is clear.
Line a small Pyrex loaf pan, terrine mould or bowl with cling film, allowing a generous amount to hang over each side. Layer the fish pieces and vegetables in the mould, arranging them so that there is an attractive mix of colours and textures. Pour the gelatine mixture evenly over the fish.
Fold the excess cling film back over the surface of the terrine and place a lid or plate on top to weigh it down. Refrigerate for 6-8 hours or until completely set.
To serve, use the excess cling film to lift the fish terrine out of the mould and turn out on a serving plate, removing the cling film. Put the shallot, lemon zest and parsley in a small bowl and stir to mix. Sprinkle the mixture over the terrine, pressing it down lightly. Serve in slices with a leafy salad.