Place the white fish in a saucepan with the milk, a few parsley stalks, nutmeg and a bay leaf, before bringing to the boil. Reduce heat and poach gently, then remove from the heat.
Melt the butter in a saucepan and slowly add the flour, stirring constantly until a roux is formed and turns lighter in colour.
Strain the milk from the fish and slowly add it to the roux, stirring constantly until creamy and smooth. Gently cook for 10 minutes until the mixture no longer tastes raw. Leave to cool.
