Heat a few tablespoons of water in a soup pot over medium-high heat. When the water begins to sputter, add the onion, bell peppers, celery and sweet potato, and cook, stirring for five minutes, adding a little water as needed.
Stir in the tomatoes, broth or water, beans, corn, chili powder, oregano, garlic and cumin. Reduce the heat to medium-low. Cover and cook, stirring occasionally, for 25 minutes, or until vegetables are tender.
Stir in the greens. Serve as is or garnished with chopped cilantro, hot sauce, scallions and/or avocado.