Place the ground beef in a medium bowl, then mix in half of the salt and half of the pepper. Shape it into balls using approximately 1 tablespoon each.
Slice the cabbage into small strips.
In the slow cooker, combine the chicken breast, meatballs, diced tomatoes, cabbage, all the seasonings, and the stock.
Cook for 6-7 hours on low or 3-4 hours on high.
When serving, ladle the soup into bowls and if desired, garnish with sliced avocado, lime juice, cilantro, sour cream, and shredded Mexican cheese.
