1Place chicken, water, garlic, onion, and salt in a large saucepan. Bring to a boil. Reduce heat; simmer 45 minutes or until tender. Remove chicken from broth, reserving broth. Set aside.
2For the Tlatonile Sauce, toast chilis, sesame seed and pepitas in large deep skillet on medium heat until fragrant, about 1 minute. Immediately remove from heat and place in blender container. Add reserved broth; cover. Blend until smooth.
3Heat oil in saucepan on medium heat. Add chili mixture; cook and stir until thickened to consistency of heavy cream, about 5 minutes. Add chicken and toss to coat. Serve with rice and beans and garnish with additional sesame seed, if desired.