Prepare the ingredients by chopping the onion and bell peppers and mincing the garlic.
Combine all ingredients except for toppings in the slow cooker: onion, bell peppers, garlic, tomatoes (with juices), broth, taco seasoning, and chicken breasts.
Cover and cook on low for 6 hours or high for 3 hours.
Remove chicken and shred with two forks; return to the slow cooker.
Stir gently and cook on low for an additional 30 minutes.
Notes
For a spicier soup, add more taco seasoning or include diced jalapeƱos.