Gently boil the fish carcasses for 15 minutes, then separate the pulp from the bones, keeping both aside: you need about 300-350 g of fish pulp.
Boil the milk.
In the meantime, toast the flour in the butter in another saucepan, then gradually add the boiling milk, stirring.
Add the fish pulp and simmer for 10 minutes, seasoning with salt: spread onto a steel pan and cover with cling film and leave in the fridge overnight to stabilise.
Use your hands to form 'fishballs', toss them in breadcrumbs, and keep them refrigerated until use.