Rinse and dry your chiles. Place under a high broiler on a broiler safe tray around 6 inches from the broiler. Keep a close eye on the chiles, and every few minutes use a pair of tongs to turn them for even broiling. You want the skin to be charred and this should take between 10-15 minutes but please adjust the time as necessary.
Once the chiles have been evenly charred, place the chiles in a plastic ziploc type bag and seal. Allow to steam for around 15 minutes.
After they have steamed, place the chiles in a dish or bowl and allow to cool. Remove the papery skins from the chiles. You can use your hands for this if you like.
Remove the stem and the seeds. I find that placing the chiles under running water helps to remove the seeds. Then chop the chiles and set aside for now.
Add olive oil to a saucepan on medium heat on your stove. Once the oil is heated, add the minced garlic and onion. Sauté for around 3 minutes until the onion has softened. Now add the flour, and stir for 1 minute.
Add the chicken broth and stir it in. Now add the chiles and the salt and pepper. Optional add ins are a bit of cumin and/or oregano! I also highly recommend adding some Hatch Green Chile Powder to increase the depth of green chile flavor! If you are using any of these herbs and spices, then add them now.
Bring to the boil, and then simmer for 15 minutes or until the sauce is thickened to your liking. Taste and add additional salt and pepper if needed.
OPTIONAL: For further thickening and a smoother sauce, add the sauce to your blender and blend until it's the consistency you like!