Preheat oven to 400° F.
Toss mozzarella, tomatoes, 2 Tbsp. olive oil, and balsamic vinegar in a medium bowl. Season with salt and pepper.
Brush each half of bread with 1 Tbsp. olive oil and season with salt and pepper. Bake on a lined baking sheet until slightly toasted, about 6-8 minutes. Remove from oven.
Brush 3 Tbsp. pesto on each half of bread. Divide the mozzarella mixture on top of the pesto on each bread half.
Bake for 7-9 minutes until cheese starts to melt and the tomatoes are warm throughout. Remove from oven and slice each half into six pieces.