First make the pastry. Sift the flour and turmeric into a bowl and, using a fork, mix with the mashed potato until blended. Rub in the margarine, then draw together into a dough, adding trickles of cold water if necessary to bind the dough together. Wrap the dough in plastic wrap and chill in the refrigerator for about 20 minutes.
Meanwhile, prepare the filling. Cut the fish into small chunks and put into a pie dish of about 1.25 litre capacity. Mix in the shrimp or seafood, leeks, peas, parsley, thyme and lemon zest.
Blend the cornstarch with a little of the milk in a bowl. Heat the rest of the milk until it is almost at boiling point, then stir it into the cornstarch paste. Return the mixture to the pan and cook, stirring, until the sauce thickens. Season to taste and pour over the fish filling. Set aside to cool for 15 minutes. Preheat the oven to 400°F (200°C).
Roll out the potato pastry between two large sheets of plastic wrap to about the size and shape of the top of the pie dish. Remove the top sheet of plastic and use the second sheet to help you lay the pastry on top of the filling. Remove the sheet of plastic wrap.
Press and trim the pastry edges to neaten, and make a small slash in the centre of the lid to allow steam to escape. Brush the pastry with the milk, then sprinkle the top with the breadcrumbs. Place the pie dish on a baking tray and bake for 10 minutes, then reduce the heat to 350°F (180°C) and bake for a further 15 minutes until golden on top.
Variation: Alternatively, top the pie with mashed potatoes flavoured with chopped fresh parsley or chives. Bake for 20 minutes at 400°F (200°C).
Per serving: 601 calories, 22 g fat (4.5 g saturated), 46 g protein, 51 g carbohydrate, 5.5 g fibre