This is a great chicken recipe when you're in a hurry; fast, easy and absolutely delicious. I normally serve this on the side with jasmine rice and red peppers, or with a cold orzo salad (search orzo salad).
In a medium bowl, whisk balsamic, onion, garlic, dijon and herbes until smooth. Slowly whisk in oil. Place chicken in sealable plastic bag, add the marinade, massage to blend ingredients, and leave at room temperature for 1/2 hour. Meanwhile, prepare the rest of your meal.
Preheat oven to 350. Remove breasts from marinade and discard. Place breasts on rack in a roasting pan and bake for 45-60 minutes until done. Tent with foil for 10 minutes, sprinkle chives over top and tomatoes on the side (if you wish), and serve.