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. Transfer steak to cutting board once it reaches the desired temperature and let rest for 3 minutes before serving.
Toss arugula with enough dressing to coat. Thinly slice steak diagonally across the grain. Serve steak over salad sprinkled with Roquefort.