1. Combine vinegar, mustard, maple syrup, 1/8 teaspoon of the salt and 1/8 teaspoon of the pepper in a small bowl. Slowly whisk in olive oil until well blended. Set dressing aside.
2. Heat a grill or grill pan on medium-high heat. Season steak on both sides with remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Brush grill lightly with vegetable oil, then sear steak for 2 to 3 minutes on one side; flip and sear another 2 minutes for medium-rare. Place steak on a cutting board. Cover with aluminum foil and allow to rest for 5 minutes. Slice steak against the grain into thin strips.
3. Arrange steak over arugula, onion and tomatoes. Sprinkle with cheese. Drizzle with dressing or serve on the side.