Heat olive oil in a medium saucepan over medium heat. Add in garlic and onion and saute for 2 minutes. Whisk in masa harina or flour until well blended. Stir in remaining ingredients and bring mixture just to a gentle boil, then reduce heat to low and simmer 20 minutes (to see an example of how to prepare the enchiladas you can view my post for Chicken Enchiladas with Green Sauce). Store any remaining enchilada sauce in refrigerator up to 1 week.