Season the chicken breasts on both sides with ½ teaspoon kosher salt and pepper.
Heat a dutch oven over medium-heat. Once hot, add the oil and sear the chicken on both sides. Do not crowd the pan. You may need to do this step in batches. Place the seared chicken on a plate, set aside.
Add the garlic, chili powder, cumin, onion powder, and the remaining 1 ½ teaspoons salt into the pot and cook until fragrant, 30 seconds.
Reserve 2 cups of chicken stock for later and add the rest to the pot and scrape the bits off the bottom of the pot.
Add the browned chicken breasts back to the pot and bring to a boil, reduce heat to a simmer and simmer for 30 minutes covered so the chicken can finish cooking.
Take the chicken out of the pot and thinly slice it. Add it back to the pot.
Mix together the remaining 2 cups of chicken stock and the masa harina (corn flour) mixture until smooth. Slowly stream in this mixture into the pot while whisking.
Mix in the enchilada sauce. Bring back to a simmer and simmer for an additional 30 minutes, stirring occasionally. The soup will thicken from the masa harina.
Turn off the heat, add in the shredded Monterey Jack cheese and cheddar cheese.
Stir it in until melted. Taste, adjust seasoning if necessary.
Serve with more of the cheeses on top, the tortilla strips, cilantro, and sour cream.
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Notes
You can use rotisserie chicken, chicken breasts or chicken thighs for this recipe.
Use your own homemade enchilada sauce if you'd like.
Other ingredients can be added in to make it heartier, see above for some options.