Preheat the oven to 350 degrees. Line a baking sheet with foil or parchment.
Coat the chicken breasts with olive oil and sprinkle them with salt and pepper. Place the chicken on the prepared baking sheet and bake for 15-20 minutes (until a meat thermometer reads 165 degrees in the thickest part of the chicken). Remove from oven. Once the chicken cools a bit, use two forks to shred and then set aside.
In a dutch oven set over medium heat on the stove, add a tablespoon or two of olive oil. Add the celery, carrots, and onion and cook until softened, about 7 minutes. Add the garlic and cook for an additional minute.
Sprinkle in some salt and pepper and add the taco seasoning. Stir around and cook for a few minutes, until very fragrant.
Add the broth, Rotel, lime juice, and shredded chicken. Allow to come to a boil and then reduce to a simmer. Let the soup simmer for an additional 10-15 minutes, stirring occasionally. Taste and adjust seasoning if necessary.
Add cooked white rice to bowls, ladle in the soup, and serve with cilantro and avocado.