Cod with parsley sauce

Ingredients

for four: 1 lemon, sliced; 700g thick cod fillet, cut into 4 equal pieces; 45g unsalted butter; 15g plain flour; 500ml whole milk; 50ml double cream; 25g parsley, finely chopped; Salt and freshly ground black pepper

Put 1 litre of water into a large pan with the lemon slices and two teaspoons of salt. Add the cod, bring to the boil and simmer for 2 minutes - slightly longer if the fillets are really thick. Remove the pan from the heat and set aside covered with foil while you make the sauce, during which time the cod will continue to cook in the residual heat.
Melt 15g of the butter in a heavy-based pan, add the flour and cook until it starts to smell nutty but is not browned. Gradually add the milk, stirring all the time, until smooth. Add 150ml of the fish cooking liquor and 50ml of cream and leave the sauce to simmer for about 20 minutes, stirring now and then, until reduced and thickened to a velvety consistency.
Finish the sauce with 30g of butter and a squeeze of lemon before stirring through the chopped parsley.