Place diced tomatoes in a medium bowl. Add onion, garlic, basil, vinegar, olive oil, salt and pepper. Let the mixture stand at room temperature for about 15 minutes and then transfer to the refrigerator for 45 minutes.
Preheat oven on broil. Brush one side of each bread slice with remaining olive oil. Toast bread on oiled sides until golden brown.
Remove from oven and spoon tomato mixture on top. Drizzle balsamic on top. Serve immediately.