Chipotle-Tomatillo Sauce

Slit chiles lengthwise, then discard stems, seeds, and veins. Heat a large heavy skillet (preferably cast-iron) over medium heat until hot, then toast chiles in batches, opened flat, turning and pressing with tongs, until more pliable and slightly changed in color, about 30 seconds per batch. Tear into pieces and put in a blender. Half-fill a 2 1/2- to 3-quart saucepan with water and bring to a boil. Add tomatillos and simmer until tender, 5 to 10 minutes. Transfer with a slotted spoon to blender and add onion, chipotle in adobo, garlic, and 3/4 teaspoon salt. Blend until smooth (use caution when blending hot liquids). Transfer to a bowl and cool to room temperature.