Cheese and tarragon soufflé
Preheat the oven to 200C/390F/Gas 6.
Brush the inside of a large soufflé dish with the melted butter and dust with the grated parmesan, shaking out any excess cheese.
Bring the milk to the boil in a saucepan with the bay leaf, then remove from the heat and set aside.
Melt the butter in a heavy-based saucepan and stir in the flour. Cook for 2-3 minutes, until the mixture turns a deep ivory colour and thickens. Pour in the warm milk, stirring or whisking until you have a smooth sauce that is free of lumps. Simmer gently, stirring from time to time, for about ten minutes.
Remove the sauce from the heat and remove the bay leaf. Whisk in the egg yolks, one at a time, and stir in the mustard and tarragon leaves. Season with salt and freshly ground black pepper. Gently stir in the Gruyère, until melted and well combined.
Beat the egg whites until soft peaks form when the whisk is removed from the bowl. Gently fold the egg whites into the cheese sauce with a large metal spoon, until just combined.
Gently spoon the mixture into the buttered soufflé dish and bake in the oven for 25-30 minutes, until the soufflé has risen and is golden-brown on top.
Remove from the oven and serve immediately.