Place the trout fillet into a saucepan of boiling water and poach for 3-4 minutes, or until cooked through. Drain and leave to cool slightly.
Flake the trout into a large bowl. Add the mustard, dill, salt and freshly ground black pepper, cream cheese and cayenne pepper and mix well.
Spoon into a ramekin and place into the fridge to set for an hour, if possible.
Garnish with a sprig of watercress and serve with toasted bread.