Method
Prep:40min  ›  Cook:20min  ›  Ready in:1hr 

1. Put the fish in a large frying pan. Tear the leaves from the parsley stalks and add the stalks to the pan with the onion, carrot, peppercorns and milk. Bring just to the boil, then cover and simmer very gently for 5 minutes. Remove the pan from the heat and leave for 5 minutes to complete the cooking.
2. Meanwhile, put the potatoes in a saucepan, cover with boiling water and simmer for 15 minutes or until tender. About 5 minutes before the end of the cooking time, add the white part of the leek to the potatoes. Also, set a colander on top of the pan and steam the courgettes with the green part of the leek over the potatoes.
3. Transfer the fish to a plate and remove the skin. Keep warm. Strain the cooking liquid and reserve.
4. Melt the butter in a medium saucepan, stir in the flour and cook for 1 minute. Gradually stir in the cooking liquid and bring to the boil, stirring until the sauce is thick and smooth. Finely chop the parsley leaves and stir into the sauce with the lemon zest. Season to taste and keep hot.
5. Drain the potatoes and white leeks and mash with the lemon juice and seasoning. Stir in the green leek tops and courgettes. Transfer the fish fillets to serving plates and spoon over the sauce. Garnish with parsley and serve with the mash and lemon wedges.