Place the chicken breasts at the bottom of your slow cooker. Add the diced onion, minced garlic, fire-roasted tomatoes, frozen corn, cumin, chili powder, and oregano on top. Pour in the chicken broth to cover all ingredients.
Cover the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours until the chicken is cooked through and tender.
Once cooked, use tongs to carefully remove the chicken breasts. Shred the chicken with two forks until it’s pull-apart tender and juicy.
Return the shredded chicken to the slow cooker and stir to combine. Taste and adjust seasoning if needed.
Add torn tortillas into the soup, stirring gently to incorporate and help thicken the broth slightly. Cover and cook on high for another 15-20 minutes, until tortillas soften and flavors meld.
Serve the hot soup in bowls, topping each with shredded cheese and chopped cilantro if desired. Let the cheese melt for a few minutes before enjoying this comforting, smoky dish.