Add chicken breast to a baking dish and season with oregano. Pour balsamic vinegar over chicken and bake at 425 for 15-20 minutes until cooked through.
While chicken cooks, add mozzarella, basil, 2 tsp oregano, optional onion, and tomato to a bowl. Brush flatbread with olive oil and put directly on the rack for 5-10 minutes.
While flatbread is getting crispy, shred your chicken and mix with the leftover vinegar in your baking dish. Add balsamic glaze to your bruschetta mixture.
When flatbread is crispy, spread 1 tbsp of pesto before adding chicken and bruschetta.
Bake for 5-10 more minutes, drizzle with extra glaze, and enjoy!