Place chicken breast, black beans, kidney beans, corn, diced tomatoes, green chilies, and onion in a large 4-6 quart slow cooker. Pour chicken broth and enchilada sauce over the top of the pot.
Sprinkle with taco seasoning, salt, and pepper. Stir to combine.
Cover and cook on low for 6 to 8 hours or on high for 4 to 6 hours. Just before serving, remove chicken from the slow cooker.
Shred with two forks, then return chicken to the soup. Stir until well mixed. Serve soup topped with cilantro and tortilla strips, if desired. Or top with other optional garnishes (see note section below).