Slow Cooker Chicken Enchilada Soup

2 boneless skinless chicken breasts, about 1 lb
2 C good-quality chicken stock
1 1/4 C or one 10 oz can red enchilada sauce*, homemade or store-bought
2-14 oz cans black beans, rinsed and drained
14 oz can fire-roasted diced tomatoes, with juice
15 oz can whole-kernel corn**, drained
4 oz can diced green chiles
2 cloves garlic, minced
1 white onion, peeled and diced
1 tsp ground cumin
1 tsp salt, or more/less to taste
optional garnishes: chopped fresh cilantro, diced avocado, diced red onion, shredded cheese, sour cream, tortilla strips/chips

Add all ingredients to a large slow cooker and stir to combine. Cook for three to four hours on high heat or six to eight hours on low heat, until the chicken is cooked through and shreds easily. Use two forks to shred the chicken. Serve warm, with optional garnishes. You can also refrigerate in a sealed container for up to five days. Or freeze it for up to three months.