Keeping white fish heads and bones in the freezer, so that you have the wherewithal to make fumet or fish stock when required, is good kitchen practice. Always rinse the bones well and cut out the gills before freezing. Most fishmongers will be happy to give you heads and bones. Any white fish - like sole, turbot, brill, cod, conger eel, monkfish and whiting - are good for stock. Avoid oily fish, like mackerel, salmon or herring. Any shellfish heads and shells are excellent additions. A good time to make fish stock is when you have crab or lobster shells after another meal, otherwise buy a hen crab and bash it up with a mallet or rolling pin.
The amounts given are for 8-10 people, because this is a special dish that lends itself to sharing on a large scale. The addition of tiger prawns or langoustines (scampi) - say 3 per person - can add a nice luxurious touch. Ask your fishmonger to fillet the fish for you in order to give you the required 1.8 kg/4 lb fillets and ask for 1 kg/2¼ lb of white fish heads and bones for the broth. Apart from the fish mentioned, conger eel, cod and even coley are also fine.
Grated Gruyère is a common addition at the table, but it is strictly optional.