Bruschetta: In a medium size mixing bowl, combine the cherry tomatoes, red onion, basil leaves, olive oil, balsamic, pinch of salt and pepper. Toss to combine. Refrigerate for 20 minutes or up to 2 days to allow the flavors to marry.
Crostini: Preheat oven to 450 degrees with the rack in the top third. Arrange the crostinis on a baking sheet and drizzle with the olive oil. Bake for 5 minutes until the edges start to turn golden brown. If you are adding mozzarella, flip, top with mozzarella and bake for another 4 minutes until the cheese is melted. If you are skipping the mozzarella, flip and bake for another 3 minutes until the edges are golden brown. You don’t need to drizzle olive oil on the second side.
Assembly: Top the crostinis with the brushcetta and serve.