Combine the tomatoes, red onion, basil, olive oil and garlic in a small bowl and gently mix until incorporated. Season with a pinch of salt and freshly ground black pepper, if desired, and set aside.
Warm the grilled chicken in the microwave or on the stovetop according to package directions.
Divide the tomato mixture among the chicken breasts, and top each with 1 tablespoon Parmesan cheese and 2 teaspoons of balsamic glaze. Serve immediately.
