Place the ingredients for the sachet in a small square of muslin (cheesecloth), draw up the edges and tie tightly with kitchen string.
To make the broth, rinse and clean the fish bodies and roughly chop; clean and scrape the lobster heads and chop into 5 cm (2 inch) pieces. Cook the fish bodies, prawn shells and lobster heads with grapeseed oil in a roasting pan over medium heat until the flesh on the bodies is translucent and the juices are reduced. Add the wine and reduce by half.
Put the olive oil, onion, fennel, celery, cayenne, capsicum, garlic, leek and saffron in a large saucepan and cook over low to medium heat until the vegetables are breaking down and starting to stick. Add the tomato paste and cook for an additional 3 minutes. Add the fish bodies mixture and 3 litres (105 fl oz/12 cups) of water plus the sachet. Bring to the boil, then reduce the heat and simmer for 45 minutes. Strain, reserving bones and broth.
Put 400 g (14 oz) of the reserved bones and 1.25 litres (44 fl oz/5 cups) of the broth in a blender. Pulse a few times to break up the bones. Pass the liquid through a fine sieve and season with the salt.
Preheat the oven to 230°C (450°F/Gas 8). Heat a large ovenproof frying pan over medium heat and add grapeseed oil to coat the surface. Season the monkfish tail with salt and pepper. Sear the monkfish on one side then roast it in the oven until cooked and firm.
Meanwhile, put 70 ml (2¼ fl oz) the broth with the prawns, cockles, fennel and potato in a large saucepan and gently heat until the cockles open and the prawns are just cooked. Add the samphire at the last minute before you plate.
Put the contents of the saucepan in serving bowls. Slice the monkfish, place on top then garnish with the croutons, 5 drops of black garlic purée per serve and Espelette pepper.