Drizzle olive oil into the slow cooker. Place chicken breasts on top of oil and season each breast with salt and pepper. Top chicken breasts with onion slices, garlic, oregano, basil, rosemary, and thyme. Drizzle balsamic vinegar over seasoned breasts and pour tomatoes on top.
Cook in the slow cooker set to High until chicken is no longer pink in the center and the juices run clear, about 4 hours.