1. Preheat grill to medium heat.
2. Simmer balsamic vinegar and brown sugar over medium-low heat in a small heavy saucepan for 8-10 minutes or until dressing is reduced in half. Set aside.
3. Grill burgers according to package instructions.
4. Brush buns with olive oil and grill.
5. Spread pesto on bottom bun and top with burger, arugula, mozzarella and tomatoes.
6. Drizzle with balsamic dressing. Top with bun and serve.