Fill medium pot with water and bring to a boil over high heat. Peel onions and garlic, adding some of the garlic and some of the onion to the pot of boiling water, along with a generous handful of salt. Add chicken breast and poach on medium heat until cooked through, approx. 20 min. Once chicken is fully cooked, place in bowl and let cool. Shred chicken with two forks and set aside.
In the meantime, thinly slice the rest of the onion and quarter the tomatoes. Process the tomato, remaining garlic, chicken broth, chipotle chilis in adobo, and salt in a blender until smooth.
Heat vegetable oil in pan over medium-high heat. Add onions and fry for approx. 4 – 5 min., or until soft and starting to brown. Sprinkle with sugar and caramelize briefly, approx. 1 min. Add shredded chicken and tomato sauce and let cook over medium heat for approx. 20 min or until sauce reduces slightly. Season with salt to taste and serve with warmed tortillas, shredded iceberg lettuce, sliced avocado, and a dollop of sour cream. Enjoy!