Mince garlic, and dice onion and red bell pepper into small pieces. Partially drain diced tomatoes by pouring off excess liquid from the top of the can.
Spray slow cooker with cooking spray. Drop in diced onion, then chicken breasts, then pour 1/2 cup of enchilada sauce so that it coats the chicken.
Next, pour in diced tomatoes, spices, salt, garlic, black beans, red bell pepper, and corn. Finally, pour 1 cup of enchilada sauce over everything and give a quick stir to the top ingredients.
Turn slow cooker on "low" for 7-8 hours. Once chicken is tender, mix everything well. Use the mixing spoon to help shred or break up chicken into pieces of desired size.
Serve over corn tortillas and/or cooked brown rice. Top with avocado, cilantro, cheese, and remaining 1/2 cup enchilada sauce, as desired.