Put the tomatillos, onion and garlic on the grill (put the garlic on a metal skewer or small piece of aluminum foil to keep it from slipping through the grates.) Grill the vegetables, removing them as they are ready, until the tomatillos, turning as needed, are soft but not bursting, the onion is tender and the garlic skin is charred, 5 to 15 minutes. Cool slightly.
Peel the garlic and put in a blender with the tomatillos, onion, cilantro, chipotle chile and 1/2 teaspoon salt. Blend until well incorporated but slightly chunky.
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