In a medium bowl, toss chicken with chili powder and salt. In a large pot or Dutch oven heat oil over medium heat. Add chicken and cook, stirring, until no longer pink, about 5 minutes. Add onion and garlic and cook until softened, 5 minutes more. Add broth, tomatoes, beans, and mixed vegetables. Bring to a boil, then reduce the heat to simmer gently until the vegetables are cooked, about 15 minutes.

Serve soup with avocado, sour cream, and crushed tortilla chips.