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.