Microwave onions, chipotle, garlic, tomato paste and oil in bowl, stirring occasionally until onions are softened, about 5 minutes. Transfer to a slow cooker. Stir in broth and chickpeas. Season chicken with salt and pepper and nestle into slow cooker. Cover and cook until chicken is tender, about 3 to 4 hours on low.
Remove chicken to a cutting board, let cook slightly then shred into bite-size pieces.
Stir in shredded chicken and tomatoes and let sit until heated through, about 5 minutes. Stir in cilantro and season with salt and pepper.
Serve with sour cream, Monterey jack cheese and avocado.