Creamy fish gratin
Preheat the oven to 190°C/375°F/Gas mark 5.
Melt 25g/1oz of the butter in a pan. Add the leeks and cook, stirring, for 4–5 minutes until golden. Pour in the stock, vermouth and Pernod, and bring to the boil. Add the monkfish and salmon, and simmer for 2–3 minutes, then add the prawns and scallops, and simmer for another 3 minutes. Remove the fish, reserve the liquor and wipe out the pan.
Add 50g/2oz of the butter and the onions to the pan, and cook for 10 minutes or until soft and golden. Stir in 50g/2oz of the flour, and cook for a further minute, stirring. Pour in the reserved poaching liquor and the cream, whisking to prevent lumps forming. Bring to the boil and bubble hard for 10 minutes. Season with salt and pepper. Put the fish and leeks in a 2.4-litre/4pt ovenproof dish, then pour the hot sauce over the top.
Melt the remaining butter in a large pan, add the remaining flour and cook, stirring, for 1 minute. Pour in the milk and, whisking continuously, bring the mixture to the boil. Stir in 100g/4oz of the cheeses and the mustard. Cool slightly, then stir in the egg yolks. Season well.
Whisk the egg whites in a bowl until they form soft peaks, then fold them into the sauce. Spoon the topping over the fish, and sprinkle with the remaining cheeses and the breadcrumbs. Cook for 1 hour and serve hot.