Preheat your oven to 200°C. Spray a large frying pan with low-calorie cooking spray, and set on a medium heat. Season the chicken breasts, to taste, with salt and pepper. Add the chicken to the pan, and fry for 6 minutes on each side — until browned. Remove the chicken from the pan and set aside.
Using the same frying pan (don't wipe), fry the onion and garlic, until the onion starts to soften — about 5 minutes.
Add the balsamic, tomatoes, and courgette, and cook for 5 minutes — stirring frequently.
Add the basil and return the chicken to the pan.
Put the pan in the oven (or in an oven-proof dish if your pan isn't suitable), and cook for 15 minutes.
After 15 minutes, place the cheese on top of the chicken and cook for a further 10 minutes. The chicken should be cooked through, with no sign of pinkness, and the juices should run clear.
Serve with greens and your choice of pasta, rice, or cous-cous.