In a medium nonstick skillet, heat the oil over medium heat. Add the onion and garlic and cook, stirring until soft, about 3 minutes. Add to the slow cooker along with the broth, tomato sauce, chipotle in adobo, cilantro, beans, tomatoes, corn, cumin, and oregano. Add the chicken breasts. Cover and cook on low for 4 to 6 hours.
Remove the chicken, shred it with two forks, and return it to the slow cooker.
To serve, ladle into serving bowls and divide evenly. Top with cheddar cheese, scallions, cilantro, avocado, and sour cream (if using).
