Heat grill to high and oil grates.
Sprinkle steak with salt and pepper.
In small blender or food processor, combine vinegar, shallot, mustard and olive oil to make dressing. Season with salt and pepper, and puree until smooth.
Grill steak until it reaches 130 degrees for medium-rare, about 6–7 minutes per side. Transfer to cutting board and let rest 5 minutes.
In large bowl, combine watercress, salt, pepper and enough dressing to coat. Thinly slice steak across the grain and transfer to platter. Top with watercress, Stilton/blue cheese and grapes, and serve with additional dressing.
Nutritional information per serving: 376 calories; 29 g protein; 10 g carbohydrates; 24 g fat (57% calories from fat); 8.5 g saturated fat (20% calories from saturated fat); 73 mg cholesterol; 299 mg sodium; .8 g fiber.
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