Whisk together mustard, sugar, lemon juice, vinegar, oil, 1/4 teaspoon salt, and 1/8 teaspoon pepper.
Season steak with 1/2 teaspoon salt and 1/4 teaspoon pepper. Grill steak, turning halfway through, 4 to 6 minutes total for medium-rare. Transfer steak to a cutting board and let rest 3 minutes.
Toss arugula with enough dressing to coat. Thinly slice steak diagonally across the grain. Serve steak over salad sprinkled with Roquefort.
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