Preheat the oven to 400°F (200°C).
Lightly grease a 9x13-inch baking dish or similar oven-safe pan.
Pat the chicken dry with paper towels. In a bowl, toss chicken with olive oil, salt, pepper, Italian seasoning, and garlic powder until coated. Arrange in a single layer in the baking dish.
Make the bruschetta topping: In a large bowl, combine diced tomatoes, minced garlic, red onion, balsamic vinegar, olive oil, salt, pepper, and chopped basil.
Stir gently. Taste and adjust salt and vinegar if needed.
Spoon about half of the bruschetta mixture over the chicken. Reserve the rest for topping after baking to keep the flavors bright.
Bake for 18–22 minutes for breasts (12–18 minutes for small thighs), depending on thickness.
The chicken is done when the internal temperature reaches 165°F (74°C).
Remove the dish from the oven. Sprinkle mozzarella and Parmesan over the chicken, then spoon on half of the reserved fresh bruschetta mixture.
Return to the oven and bake for another 4–6 minutes, just until the cheese melts and the tomatoes warm through. For a golden top, switch to broil for the last 1–2 minutes, watching closely.
Let rest for 5 minutes.
Top with the remaining fresh bruschetta, a few torn basil leaves, and a light drizzle of balsamic glaze if you like.
Serve with pasta, rice, quinoa, zucchini noodles, or toasted bread to soak up the juices.