Pour the oil and vinegar into a large plastic zip-top bag.
Add in the brown sugar, the black pepper, the smashed garlic, the onion slices, and the sprigs of rosemary and thyme.
Seal bag, and "knead around" to mix well.
Put the chicken breasts into the bag with the marinade; reseal bag and knead it around to coat all the surfaces of the chicken. Place the bag into the refrigerator for 2-4 hours. (Occasionally knead bag).
When you have your coals ready on your BBQ grill, take out the sprigs of rosemary, and thyme and lay them directly on the top of the coals. Place chicken on an oiled grill rack over the heat. Grill 5-8 minutes per side, or until chicken is done.