1. Heat olive oil in a large skillet over medium heat.
2. Add sliced onions and cook until translucent.
3. Add minced garlic and cook for another minute.
4. Add diced tomatoes, chipotle peppers, oregano, and cumin. Cook for 5 minutes.
5. Transfer the mixture to a blender and blend until smooth.
6. Return the sauce to the skillet and add chicken broth.
7. Add chicken breasts to the skillet and bring to a simmer.
8. Cover and cook for 20 minutes, or until chicken is cooked through.
9. Remove chicken from the skillet and shred with two forks.
10. Return shredded chicken to the skillet and mix with the sauce.
11. Season with salt and pepper to taste. Serve hot.