Cut the fish into pieces and place in a saucepan with the milk, slice of lemon, bay leaf, parsley stalks and peppercorns. Cover with a lid and simmer gently for about 15 minutes to cook the fish. Strain off the cooking liquor and reserve it for the sauce. Allow the fish to cool for a few moments, then carefully remove the skin and small bones and flake the flesh.
Melt the butter in a sa