Place the milk, parsley stalks, bay leaf, sliced onion and whole black peppercorns in a small pan, bring everything slowly up to simmering point, then pour the mixture into a bowl and leave aside to get completely cold.
When you're ready to make the sauce, strain the milk back into the pan, discard the flavourings, then add the flour and butter and bring everything gradually up to simmering point, whisking continuously with a balloon whisk until the sauce has thickened and is smooth and glossy. Then turn the heat down to its lowest possible setting and let the sauce cook gently for 5 minutes, stirring from time to time.
To serve the sauce, add the chopped parsley, cream and lemon juice,taste and season, then and pour over the Cod