Method

For the gratin, bring the stock to a simmer in a large saucepan. Add the fish and poach for 1 minute. Add the scallops and poach for another minute. Add the prawns and poach for a final minute. Remove all the seafood with a slotted spoon and set aside. Add the wine, bring to the boil and reduce for 5 minutes. Then add the milk and cream and reduce to a simmer.
Meanwhile, melt the butter in another large saucepan, add the leeks and cook for about 5 minutes until soft. 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 with no lumps. Bring the sauce to the boil, turn down the heat and leave to simmer for about 15 minutes. Preheat the grill to high.
Grease six shallow ovenproof gratin dishes or one large baking dish measuring about 30x25cm/12x10in. Add the poached fish, scallops and prawns to the leek sauce and season with salt and pepper. Stir in the parsley and transfer the mixture to the gratin dishes or dish.
Mix the cheese with the breadcrumbs and sprinkle over the top of the gratin. Place under the hot grill for 10 minutes until golden-brown and the contents are bubbling.
For the caramelised apples, melt the butter in a non-stick frying pan. Add the caster sugar and stir until the sugar has dissolved. Add the apple chunks and stir to coat them in the caramel. Cook for about 10 minutes, or until the apples have softened and are covered in the caramel sauce.
Serve the gratin with the caramelised apples and bread on the side.