Combine the diced tomatoes, red onion, garlic, basil, olive oil, balsamic vinegar, salt, and pepper together in a large bowl.  Sprinkle with the Parmesan, if desired.

Heat a broiler or grill to high heat, then toast the baguette slices on each side until lightly browned.  

Serve together for guests to top their toasts individually so they don't get soggy, or top each toast with the tomato mixture and serve immediately.

If desired, make a balsamic reduction to drizzle over the top of each bruschetta by bringing the balsamic vinegar to a boil over medium heat in a small saucepan, then reducing the heat to medium-low until the liquid has reduced by half, about 5 minutes.  Remove from heat and allow the balsamic vinegar to cool before drizzling over the bruschetta.  The reduction will continue to thicken as it cools.