Make the vinaigrette: Whisk together the vinegar, mustard, salt and pepper in a medium bowl. While whisking, slowly drizzle in the oil and whisk until emulsified. Set aside.
Place the steak in a baking dish. Add half the vinaigrette and turn to coat; let the steak sit at room temperature for 30 minutes.
Heat a grill to high or a grill pan over high heat on top of the stove.
Remove the steak from the marinade and pat dry. Season both sides with salt and pepper. Grill until lightly charred on both sides and just cooked to medium-rare doneness, about 4 minutes per side. Transfer the steak to a cutting board and let rest for 5 minutes.
Meanwhile, mix the arugula and red onion in a large bowl. Add some of the remaining vinaigrette and toss to coat. Season with salt and pepper.
Thinly slice the steak across the grain and arrange it on a platter with the tomatoes. Serve with the arugula salad.