To prepare dressing, combine first 8 ingredients in a small bowl; stir well with a whisk.
To prepare praline, place sugar in a small skillet over medium heat, and cook until sugar dissolves, stirring as needed to dissolve sugar evenly. Stir in chopped walnuts, and cook over low heat 30 seconds or until golden. Remove from heat. Rapidly spread mixture onto foil coated with cooking spray. Cool completely; break into small pieces.
To prepare salad, combine watercress, lettuce, and blue cheese in a bowl; add dressing and praline, tossing well. Serve immediately.