Place tomatillos in a small saucepan, and add enough water to come about half of the way up. Cover, and cook over medium heat until tomatillos are soft, about 10 minutes. Strain, reserving liquid. Add enough water to the reserved liquid to make 1/2 cup.
Meanwhile, heat a small heavy skillet or a comal over medium heat until hot. Add garlic to pan, and cook until soft and well charred on all sides, 5 to 10 minutes. Peel, and transfer to jar of a blender. Add chiles to pan, and toast for 2 minutes per side. Tear chiles into pieces, discarding stem; transfer to blender jar. To the blender, add cooking liquid and a large pinch of salt. Blend until smooth. Add tomatillos, and blend. Will keep in an airtight container in the refrigerator for up to 1 week.