Preheat a grill to high and oil the grates. Sprinkle the steak with salt and pepper.
In a small blender or food processor, combine the vinegar, shallot, mustard and olive oil. Season with salt and pepper and puree until smooth. Grill the steak until it reaches 130 F for medium-rare, about 6-7 minutes per side. Transfer to a cutting board and let rest 5 minutes.
In a large bowl, combine the watercress, salt and pepper and enough dressing to coat. Thinly slice the steak across the grain and transfer to a platter. Top with the watercress, Stilton and grapes and serve with additional dressing.
Nutritional information per serving: Calories 376; Protein 29 g; Carbohydrate 10 g; Fat 24 g (57% Calories from Fat); Sat Fat 8.5 g (20% Calories from Saturated Fat); Cholesterol 73 mg; Sodium 299 mg; Fiber .8 g.