First clarify the butter. Heat a small saucepan of water, add the butter and heat gently until melted. Cool, then freeze until the butter has formed a set layer on top of the water. Lift the disc of hardened butter off the water and discard the water. Remove any droplets of water from the underside of the butter with kitchen paper.
Melt half the clarified butter in a saucepan. Add the lime rind, chopped coriander, cayenne and a little salt. Stir the prawns into the butter with the crab meat and lime juice. Heat until piping hot. Spoon into 4 small china ramekins and press down well so there are no air pockets. Chill for 15 minutes.
Melt the remaining butter in a clean pan then spoon over the top of the fish mixture in a thin even layer. Chill for 3–4 hours until set. Sprinkle with a little extra cayenne.
When ready to serve, toast the bread then arrange on plates with the dishes of potted fish. Serve with chicory leaves and tiny radishes.