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.
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 zucchini with the green part of the leek over the potatoes.
Transfer the fish fillets to a plate and carefully remove the skin. Keep warm. Strain the cooking liquid and reserve.
Melt the margarine in a medium saucepan, stir in the flour and cook for 1 minute. Gradually stir in the reserved 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 along with the lemon zest. Season to taste and keep hot.
Drain the potatoes and white leeks and mash with the lemon juice and seasoning. Stir in the green leek tops and zucchini. Transfer the fish fillets to serving plates and spoon over the sauce. Garnish with parsley and serve with the mash and lemon wedges.
Some more ideas…
Cod and English spinach gratin: Cook the fish and potatoes as described in the main recipe. Slice 3 tomatoes and arrange in a shallow flameproof dish. Steam 300 g English spinach leaves until wilted, then squeeze dry and sprinkle with 1 tbsp lemon juice. Spread over the tomatoes and top with the fish. Pour over the parsley sauce and spread over the mashed potatoes. Sprinkle with 2 tbsp grated mature tasty cheese and grill until golden.
*Fish provides excellent quantities of iodine. Fish is one of the most reliable sources of this essential mineral because of the consistent iodine content of sea water. Other foods depend on the iodine in soil, which can vary. *Zucchini provide niacin and vitamin B6. It is the tender skins that contain the greatest concentration of these nutrients.