Prep the pan: Heat the oven to 425°F (220°C). Line a large sheet pan with parchment paper or lightly grease it with olive oil.
Season the chicken: Pat the chicken breasts dry with paper towels.
Rub with 1 tablespoon olive oil, 1/2 teaspoon kosher salt, and 1/4 teaspoon black pepper. Sprinkle with Italian seasoning.
Toss the tomatoes: In a bowl, combine halved tomatoes, minced garlic, remaining 1 tablespoon olive oil, balsamic vinegar, remaining 1/2 teaspoon salt, remaining 1/4 teaspoon pepper, and red pepper flakes if using. Add sliced red onion if you like.
Arrange on the pan: Place chicken on the sheet pan with a little space between each piece.
Scatter the tomato mixture around and slightly on top of the chicken.
Roast: Bake for 15–18 minutes, depending on the thickness of the chicken. The tomatoes should be soft and juicy.
Add cheese (optional): If using mozzarella and Parmesan, top each chicken breast with cheese and return to the oven for 2–4 minutes, just until melted and bubbling.
Check doneness: The chicken is done when it reaches an internal temperature of 165°F (74°C). If needed, roast another 2–5 minutes.
Finish and serve: Sprinkle with fresh basil and a light drizzle of balsamic vinegar.
Add a squeeze of lemon if you like. Serve with toasted baguette, rice, or a simple green salad.