Rinse the garbanzo beans; place in a large saucepan. Add enough water to cover the beans by 2 inches. Bring to boiling; reduce heat. Simmer, uncovered, for 10 minutes. Remove from heat. Cover and let stand for 1 hour. (Or place beans in water in a large saucepan. Cover and let soak in a cool place overnight.) Drain and rinse beans.
Rinse chicken. Place chicken and beans in a 3 1/2-, 4-, or 5-quart crockery cooker. Add carrots, fennel or celery, onion, dried marjoram and thyme (if using), bouillon granules, salt, and pepper. Pour water over all.
Cover; cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours. Remove the chicken; cool slightly. Cut the meat into bite-size pieces. Return to cooker. Add the spinach or escarole and, if using, the fresh marjoram and thyme. Let stand 5 minutes before serving.
To serve, ladle soup into bowls. If desired, garnish with fresh thyme sprigs.