Heat a medium size pot on medium heat and add olive oil.
Add the onions, celery, Serrano pepper, and garlic. Stirring as they cook for 5 minutes.
To the pot, add the pieces of chicken and stir. Continue to sauté the chicken and vegetables while stirring until the chicken is thoroughly cooked (about 8 minutes or it may take longer depending on the thickness of the chicken pieces).
Then add the chicken broth, oregano, cumin and pepper, and stir.
Blending the Roasted Green Chiles - The next step is to blend the roasted green chilis with some of the broth from the pot.
When using a blender for hot liquids it’s very important to use caution; since the ingredients are hot, steam can build up and blow the lid off of the blender.Here are some tips when using a blender for hot liquids:- add the soup to the blender jar in batches that are no larger than 1/3 full- remove the center piece from the blender lid- place the lid (minus the center piece) on the blender jar- cover the lid with a clean towel folded to a size slightly larger than the lid- hold the towel over the lid while blending on the lowest speed
In a blender, add the roasted green chiles, tortillas, and about 3 ladles of broth from the pot. Blend to incorporate all of the ingredients.
Add the green chile mixture to the pot and stir. Add the cilantro stems (reserving the cilantro leaves to add just before serving) and stir. Add salt and stir. Give it a taste test and add more salt if needed.
Cover the pot with a lid and turn the heat down to simmer. Let simmer for another 15 to 20 minutes.