Make a slit in each of the ancho chiles and New Mexico chiles with a sharp knife, and remove the seeds and stem.
Place the chiles in a large saucepan, and add the chicken stock, onion, and garlic. Bring the mixture to a boil over high heat, then reduce heat to medium, and simmer until the chiles have softened, about 15 minutes.
Pour the chile mixture into a blender, filling the pitcher no more than halfway full. Hold down the lid of the blender with a folded kitchen towel, and carefully start the blender, using a few quick pulses to get the mixture moving before leaving it on to puree. Puree in batches until smooth and pour into a clean pot. Alternately, you can use a stick blender and puree the chile mixture right in the cooking pot.
