In a large cast iron skillet (10 to 12 inch diameter), saute the onion in the olive oil over medium-low heat until translucent. Add the chicken and cook for about 10 minutes, until the chicken is cooked through (no longer pink in the middle). Stir regularly during the 10 minutes to cook evenly.
Add the cooked rice, tomato sauce, chicken broth, chili powder, cumin, and crushed tortilla chips to the skillet. Stir everything together and let the mixture simmer for 5 minutes to thicken. Taste the mixture and add salt and/or other seasonings as needed.
Sprinkle the cheese on top of the mixture, then bake at 375 degrees for 10 minutes (or until the cheese is melted). You can turn on the broiler briefly at the end of the baking time to brown the cheese a bit.