Preheat the oven to 375°F (190°C).
In a small bowl, combine the quartered grape tomatoes, finely chopped red onion, minced garlic, and balsamic vinegar; toss and refrigerate until ready to use.
In a mixing bowl, toss the cubed chicken with the basil pesto until evenly coated.
Spread the pesto-coated chicken in a single layer in a 9x13 baking dish.
Spoon the tomato mixture over the chicken, leaving behind any excess balsamic liquid so it does not pool on the chicken.
Sprinkle the shredded mozzarella evenly over the tomatoes and chicken.
Toss the seasoned panko crumbs with the olive oil until moistened, then sprinkle the crumb mixture over the cheese.
Bake uncovered for 25–30 minutes, until the chicken is cooked through and the cheese is melted; tent with foil if the crumbs brown too quickly.
