Heat a skillet over medium heat and cook the chorizo, breaking it up with a spoon, until it is browned and releases some fat, about 5 minutes.
Add the diced onion and minced garlic to the skillet and cook for 2 more minutes until the onion softens.
Transfer the chorizo mixture to a slow cooker. Add the black beans, fire-roasted tomatoes, chicken broth, cumin, smoked paprika, cayenne pepper, and salt.
Stir everything together, cover the slow cooker, and cook on low for 6 to 7 hours.
Just before serving, stir in the lime juice. Taste and adjust salt if needed.
Ladle the soup into bowls and top with fresh cilantro and diced avocado.