In a large pot, heat olive oil over medium heat. Add the chopped onion and sauté for about 3-4 minutes until softened.
Stir in the minced garlic and diced bell pepper, cooking for an additional 2-3 minutes until fragrant.
Add the chicken breast to the pot, along with the diced tomatoes (with their juices), chicken broth, black beans, corn, chili powder, cumin, salt, and pepper. Bring the mixture to a boil.
Once boiling, reduce heat to low and cover the pot. Simmer for about 20-25 minutes, until the chicken is cooked through.
Remove the chicken from the pot and shred it using two forks. Return the shredded chicken back to the pot, stirring to combine everything.
Taste the soup and adjust seasoning if necessary. Let it simmer uncovered for another 5-10 minutes to thicken slightly.
Serve the soup hot, garnished with shredded cheddar cheese, diced avocado, fresh cilantro, and tortilla strips on top.
Notes
Adjust the spice level by adding more or less chili powder.