Heat the fish stock and add the cleaned sea urchin.
Stir well until the sea urchin is fully dissolved, then season to taste with salt.
Add the pre-soaked gelatin, strain through a fine sieve, and let cool until slightly thickened.
Whip the cream until it reaches a yogurt-like consistency, then fold it into the sea urchin stock.
Portion the mixture into sakura moulds and use a skewer to poke small holes around the edges to help release the mousse.
Refrigerate, then freeze until firm and easy to unmould.
Spray the frozen mousse with orange velvet spray and store in the freezer until ready to use.