Whisk together the vinegar and mustard in a small bowl. Slowly whisk in the olive oil and season with salt and pepper to taste.
Toss the watercress in a few tablespoons of the dressing and arrange on a platter. Top with slices of the tomatoes and onions and drizzle with the remaining dressing. Sprinkle with the blue cheese.
Copyright 1999 Bobby Flay. All Rights Reserved.