Preheat the oven to gas 5, 190°C, fan 170°C. Melt 15g of the butter and grease a 1.1 litre soufflé dish.
Melt the remaining butter in a large saucepan. Take off the heat and stir in the flour, mustard powder and Worcestershire sauce. Return to the heat and gradually add the milk, stirring until thickened. Season to taste. Stir in the watercress and leave to cool for 10 minutes.
Beat in the egg yolks, then carefully stir in the salmon. Whisk the egg whites until stiff and fold into the sauce using a large metal spoon. Pour into the soufflé dish and bake for 30-35 minutes until well risen and golden. For a heartier meal, serve with crusty bread and salad.
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