Heat the olive oil in a large pot or Dutch oven over medium heat.
Add the diced onion and sauté until translucent, about 3-4 minutes. Stir in the minced garlic and cook for another 1-2 minutes until fragrant.
Add the boneless, skinless chicken breasts to the pot and sear on both sides for about 3 minutes each.
Pour in the diced tomatoes (with juice), chicken broth, tomato sauce, chili powder, cumin, paprika, onion powder, garlic powder, salt, and black pepper. Stir to combine.
Bring the soup to a gentle boil. Reduce the heat to low and cover. Let it simmer for about 20 minutes, or until the chicken is cooked through and tender.
Once cooked, remove the chicken from the soup and shred it using two forks. Return the shredded chicken to the pot.
Stir in the diced bell pepper, corn, and green chilies (if using). Let the soup simmer for an additional 5 minutes, allowing the flavors to meld.
Serve hot, garnishing each bowl with shredded cheese, sour cream, fresh cilantro, and avocado slices, if desired.