method
1. Stir the milk, Worcestershire sauce, mustard powder and cheese in a small, heavy-based saucepan over a low heat until the cheese has melted and the mixture is smooth. Remove from the heat and leave to cool and thicken for 10 minutes, stirring from time to time.
2. Toast the bread under a moderately hot grill on one side only. Spread the cheese mixture on the untoasted sides and sprinkle with paprika.
3. Turn the grill up to high and cook until the topping is golden brown and bubbly. Lift the slices onto individual plates, garnish with the watercress and onion, and serve immediately.
