Toss all of the chicken ingredients into your crock pot and cook on low for 6 hours or on high for about 4.
Once you have around 30 minutes left on the chicken, place the diced pepper and onion into a small pan. Cook on medium until the onions are transucent, about 6-8 minutes. Add the black beans and remaining spices to the pan. Cook on a low simmer for 10-15 minutes.
Once the chicken is finsished cooking, using two forks, shred the chicken in the crock pot. Leave the chicken in the crock pot and set temperature to low.
To assemble the tostadas, start with black beans then top with pulled chicken. Serve with queso fresco, cilantro, avocados, pickled red onions, and lime wedges.