Heat a Dutch oven over medium heat. Coat pan with cooking spray. Add onion, bell pepper, and garlic; cook 5 minutes or until onion is translucent, stirring often. Add chicken to pan; cook 5 minutes, stirring often. Add cilantro and next 6 ingredients (through tomatoes), stirring to combine; bring to a boil. Cover, reduce heat, and simmer 10 minutes.
Combine masa harina and 3 tablespoons water in a small bowl. Add to chicken mixture, stirring well to combine. Simmer, uncovered, 10 minutes. Garnish with chopped cilantro, if desired.