Season both sides of chicken breasts with garlic salt and pepper.
Heat olive oil in a skillet over medium to medium-high heat; cook seasoned chicken breasts until chicken is browned, 3 to 4 minutes per side.
Add onion and green pepper; cook and stir until onion is translucent and pepper is softened, 3 to 4 minutes.
Pour diced tomatoes and balsamic vinegar over chicken; season with basil, oregano, rosemary, and thyme. (I like to mix them in a small bowl first, then sprinkle on.)
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).