Add oil, spices, and onion to a saucepan and sauté for about 5 minutes. Toasting deepens the flavor of the spices and helps everything blend well.
Add chicken broth, tomato sauce, fresh oregano, and green chilies. Cook on a slow simmer for at least 30 minutes, stirring occasionally.
Divide into thirds and either use immediately or store for later. I freeze mine in mason jars, about 2 cups each, and use the remaining portion in my recipe.
P.S. Most enchilada sauces call for adding flour or another thickener to make a heavier sauce. I feel this isn’t necessary, but it can be added if needed depending on your preference.