Preheat your oven to 375 F (190 C). Lightly grease a baking dish or line it with parchment for easier cleanup.
Prepare the chicken by cutting a pocket into the side of each breast. Be careful not to cut all the way through.
Season the inside of each pocket with a pinch of salt and pepper. This is a quiet step, but it makes a big taste difference.
Layer mozzarella and tomato slices inside each pocket. Add a few basil leaves for brightness. If you like, you can also add a whisper of garlic inside the pocket.
Secure the opening with toothpicks if needed, then brush the outside of each chicken breast with olive oil and sprinkle with Italian seasoning, salt, and pepper.
Arrange the stuffed breasts in the prepared dish. Drizzle any remaining olive oil over the top.
Bake for 25 to 30 minutes, or until the chicken is cooked through and the cheese is melted. If you have a meat thermometer, aim for 165 F (74 C) at the thickest part.
If you want a golden finish, turn on the broiler for the last 2–3 minutes of cooking, watching closely so the cheese doesn’t burn.
Remove from the oven and let rest for a few minutes. Finish with a scattering of fresh basil and a light drizzle of balsamic glaze, if you like.