Boil the tomatillos and the chilis in a cup of water for 15 minutes. Liquify the chilis, the tomatoes, the tomatillos, the garlic and add the vinegar. Strain.
Fry the onion; add the above mixture and cook 10 minutes more, stirring constantly. Add salt and serve with warm tortillas.
Often served with queso aņejo sprinkled on top (Romano cheese or dry or aged ricotta may be substituted).
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