Season both sides of the chicken breasts with garlic salt and pepper, rubbing the spices in lightly to make them stick.
Heat a skillet over medium heat and add the olive oil and seasoned chicken breasts when it reaches temperature. Brown the chicken on each side, usually about 3 to 4 minutes per side.
Slice the onions to thin slivers and add to pan. Cook and stir until onion is browned, 3 to 4 minutes.
Reduce heat to medium low and pour diced tomatoes and balsamic vinegar over chicken, then add the rest of the spices: basil, oregano, rosemary and thyme. Cover the pan with a lid and simmer until chicken is no longer pink and the juices run clear, about 15 minutes or until internal temperature reaches 165* F (74* C).