In a small bowl, mix flour with all of the spices and salt. Set aside.
Heat 2 tablespoons olive oil in a large flat skillet or a sauce pot and add finely diced onions. Cook over medium heat until onions are translucent and just beginning to brown, stirring occasionally.
Reduce heat to medium-low and add the remaining tablespoon of olive oil and the flour-spice mixture. Whisk into onions and cook along with onions for about 5-7 minutes to bloom the spices and slightly toast the flour, stirring often. This mixture might be crumbly or pasty, depending upon the moisture in the oil and the onions, but either way it’s ready to add the liquid.
Slowly add 1 1/2 cups of the broth and whisk into the flour-spice mixture until integrated, stirring all the while to prevent lumps. At first this will look like a paste, but as you add the liquid it will thin out and become more saucy. Bring to a low boil. At this point, you may need to add a little more broth to thin out the sauce to desired consistency.
If you make your enchilada sauce ahead of time, you may need to thin it out with some water or additional broth before you pour it over your enchiladas for baking.