Season both sides of chicken breasts with garlic salt and pepper.

Heat olive oil in a skillet over medium heat; cook seasoned chicken breasts until chicken is browned, 3 to 4 minutes per side. Add onion; cook and stir until onion is browned, 3 to 4 minutes.

Pour diced tomatoes and balsamic vinegar over chicken; season with basil, oregano, rosemary and thyme. Simmer until chicken is no longer pink and the juices run clear, about 15 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).