First, make the fish velouté sauce. Melt the butter in a heavy-based saucepan over low heat then allow to cool for a minute. Add the flour and combine to make a roux. Return to a very low heat, stirring continuously for 3 minutes; it must cook slowly. Allow to cool for 5 minutes.
In separate pans, bring the milk and stock to the boil. Off the heat, add the milk