De-stem, de-seed, and de-vein the chiles. Toast both sides of each chile on a hot comal, pressing down with a metal spatula. Be careful not to burn them. It should only take a few seconds on each side.
Add the chiles to a small pot of water and bring to a boil. Remove from heat and set aside for 20 minutes to re-hydrate the chiles.
While the chiles soak, toast the white onion slices on the comal until they start to blacken. About 4-5 minutes per side. Set aside. Toast the unpeeled garlic cloves until blackened. About 5 minutes.
Strain the chiles through a fine mesh sieve. Peel the garlic, then add it, along with all of the other ingredients to your blender and puree on high speed until it is as smooth as you can get it. Strain into a pan (if you have a high-power blender, like a Vitamix, you can skip straining it).
Cook over medium-low heat for an additional 15 minutes.