To prepare the fish: Place the stock or water in a large saucepan, add the bay leaf and bring to the boil. Add the fish, bring back to the boil then reduce to a simmer and poach the fish for 3 minutes. Remove the fish with a slotted spoon and set aside. Add the wine to the saucepan, bring back to the boil and allow to reduce for 5 minutes. Then add the milk and cream, reduce to a simmer and cook for a further 5 minutes.
Meanwhile, melt the butter in another large saucepan, add the leek and onion and cook for about 10 minutes until softened but not browned. Add the flour, stir well and then add the milk and stock mixture, a ladleful at a time. Keep stirring until all the liquid is incorporated and the sauce is smooth. Bring the sauce to the boil, turn down the heat and leave to simmer for about 5 minutes. Stir through the mustard.
Preheat the oven to Gas Mark 7, 220°C (425°F).
Grease a large baking dish measuring approximately 30x20cm. Add the poached fish to the leek sauce and season with salt and pepper. Remove the bay leaf, stir in the parsley and transfer the mixture to the baking dish.
To cook: Mix the cheese with the breadcrumbs and sprinkle over the top of the mixture in the baking dish. Place in the hot oven for 10 minutes or until golden-brown and bubbling.
