For this you’ll need a 21/2 pint (1.5 litre) soufflé dish, buttered quite generously.
Start off by cooking the courgettes gently in 2 oz (50 g) of the butter until softened and beginning to colour. Remove them from the heat, season with salt and pepper and leave to cool. Now melt the remaining 2 oz (50 g) butter in a saucepan, stir in the flour and cook for a minute or two before gradually adding the milk, stirring all the time. Bring the mixture up to a simmer, still stirring, and let it cook for about 2 or 3 minutes. Take the sauce off the heat now and cool it by sitting the base of the pan in some cold water.
As soon as it’s cool enough, gradually beat in the egg yolks, followed by the cheeses, parsley and chives. Season with salt, pepper and freshly grated nutmeg. Now beat up the egg whites to the stiff peak stage and carefully fold them into the cheese mixture, using a metal spoon. Next spoon half the soufflé mixture into the prepared dish, sprinkle in the courgette slices and pour the remaining soufflé mixture on top.
Now place the soufflé dish in a roasting tin, pour a little boiling water in the tin to a depth of 1 inch (2.5 cm) and transfer the whole lot to the oven. Lower the heat to gas mark 3, 325°F (170°C), bake for 40 minutes and then increase the heat to gas mark 6, 400°F (200°C) for a further 10 minutes.