Preheat the oven to 375°F and spray an 8x8 baking dish with cooking spray.
Heat olive oil in a large skillet over medium-high heat until hot.
Add the bite-sized chicken pieces in a single layer and season with Italian seasoning, salt, and black pepper.
Cook chicken for 8-10 minutes, stirring occasionally, until golden brown and the internal temperature reaches 165°F, then remove from heat.
In a large bowl, mix diced fresh tomatoes, minced garlic, and balsamic vinegar to coat evenly.
Spread half of the tomato mixture on the bottom of the prepared baking dish.
Layer the cooked chicken over the tomatoes.
Spoon the remaining tomato mixture evenly over the chicken.
Sprinkle shredded mozzarella cheese evenly on top.
Bake for 20-25 minutes until the cheese is melted and tomatoes are softened.
Let the dish cool for 2-3 minutes, then garnish with chopped fresh basil before serving.