Lightly sauté the leeks in the butter until softened but not browned. Stir in the flour and mustard.
Gradually pour in the milk stirring constantly until the mixture has thickened. Season well and add cheese. Beat egg yolks into mix using a wooden spoon.
At this point the mixture can be placed in a covered bowl and chilled until required.
When ready to cook beat whisk egg whites in a large bowl using an electric whisk until the bowl can be inverted without the mixture falling out. Stir in a couple of spoonfuls of the egg white into the soufflé mix using a metal spoon to loosen the mix. Carefully fold in remainder to retain as much volume as possible.
Lightly grease 6 individual ramekin dishes or 1 large soufflé dish with butter. Spoon soufflé mixture into the dishes. Rest on a baking sheet.
Cook until the soufflé is risen above the dish rim and is golden and fluffy. 220C/200C Fan/425F/Gas Mark 7.
Serve immediately before it sinks before your eyes!
To make one large souffle adjust the cooking time to around 40 minutes.