Reduce heat to medium and add onions to same pot. Cook, stirring occasionally, until very soft and just beginning to brown around the edges, 8–10 minutes. Add tomatoes and cinnamon sticks. Cook, stirring occasionally, until tomatoes are softened and juices have thickened slightly, 6–8 minutes. Return chicken to pot, arranging skin side up. Cover pot, reduce heat to low, and simmer gently until chicken is cooked through, 45–60 minutes.