Preheat the oven to 200°C/ Gas 6. Melt the butter in a small saucepan, stir in the flour and cook the roux gently for 2 or 3 minutes. Pour in the warmed milk and bring the sauce to the boil, stirring constantly. Remember to scrape round the edges of the pan so that all the uncooked roux is well stirred in. When the mixture comes to the boil, let it simmer over a very gentle heat for 3 - 5 minutes, stirring occasionally. Then remove the pan from the heat and add the grated cheese and mustard.
When the sauce has cooled a little, beat in the egg yolks and then season the mixture to taste - remember that the cheese will already have made this sauce quite salty.
Add the chopped watercress to the sauce. In a China glass or copper bowl beat the egg whites, preferably with a hand whisk, until they are moist and velvet in appearance and just peak softly. Then stir in a spoonful of beaten egg white into the cooked sauce.
Fold in the remaining egg whites in two batches, cutting the mixture through with a metal spoon or palette knife.
Put this mixture immediately into a greased 900ml/1½pt soufflé dish and bake for 25 minutes or until it is well risen and firm to the touch in the centre. Serve immediately.