Place the milk, parsley stalks, peppercorns, garlic and cod into a deep frying pan. Bring to the boil and reduce to simmer for about 5 minutes or until the fish turns opaque. Remove fish gently from liquid and keep warm. Strain liquid and keep warm.
You need 21⁄2 cups of liquid to make the sauce so make this up with extra milk. Melt butter in another saucepan and stir in the flour slowly over a low heat and cook. Stirring constantly for about 3 to 5 minutes until mixture is bubbling and starts to turn golden. Gradually add the reserved milk and stir until sauce boils and thickens. Season to taste and stir in the parsley.
Boil potatoes and mash well. Serve with cod and pour parsley sauce over the top.