Start by preheating your oven to 350°F for the chicken.
Arrange the chicken breasts on a baking sheet and season with 1 tablespoon of the taco seasoning mix.
Bake the chicken for 30 to 35 minutes or until an internal temperature of 165°F is reached. Once done, let it cool and then shred.
While the chicken bakes, heat oil in a large pot over medium-high heat and sauté onions and celery until tender.
Pour in the chicken broth and water, and bring to a boil. Season with the remaining taco seasoning, cumin, and black pepper, then simmer to blend the flavors.
Add the shredded chicken, tomatoes, and cilantro to the pot and simmer for an additional 5 minutes.
Serve the soup hot, garnished with shredded cheddar cheese, crushed tortilla chips, and diced avocado.