In a large pot, add olive oil and onion on medium low heat. Cook until onion is partly cooked, about 5 minutes. Add taco seasoning and cumin, and cook for 1 minute, until fragrant.
Add tomatoes, beans, corn, and broth. Turn heat to medium high. Once it reaches a boil, reduce heat, and simmer for 10 minutes stirring occasionally.
Add chicken and spinach if using, and cook for additional 5-10 minutes.
If desired, serve with sour cream and cilantro and sprinkle with pepper.
Notes
*I used 2 cups broth. If you prefer more broth, a soupier soup, add 2 additional cups for a total of 4 cups.*Nutrition information does not include optional garnish.