In addition to being a filling for soft tacos, this chicken dish can be served with rice and a salad for a light meal.
Place chicken in a large saucepan and cover with water. Halve one of the onions and add to water. Bring to a simmer and cook for 20 minutes, or until chicken is cooked through. Remove chicken from liquid and let cool. Discard cooking liquid and onion halves. When chicken is cool, shred with your fingers and set aside. Discard skin and bones.
To make sauce, chop remaining onion. In a large saute pan over medium-low heat, cook onion in oil for 5 minutes. Add tomatoes, chiles, and salt to taste, and cook for about 15 minutes, or until a thick sauce is formed.
Add shredded chicken and cook for 5 more minutes. Serve hot.