Preparation Discard stems from chiles and remove seeds. Rinse chiles under cold running water. Bring chiles with water to a simmer in a nonreactive 1 1/2-quart saucepan. Add onion, garlic, and a pinch of salt, then simmer, uncovered, stirring occasionally (add more water if necessary to cover chiles), until softened, about 30 minutes. While chiles are cooking, finely grind cumin seeds with a mortar and pestle or in an electric coffee/spice grinder. Transfer chiles and vegetables with a slotted spoon to a blender and purée with cumin and cup chile water (save remaining chile water) until smooth. (Sauce should be thick but not stiff; add more chile water, 1 tablespoon at a time, if necessary.) Season with salt. Cooks' note:Sauce may be made 3 days ahead, cooled completely, and chilled, covered. Reheat before serving.
Preparation Discard stems from chiles and remove seeds. Rinse chiles under cold running water. Bring chiles with water to a simmer in a nonreactive 1 1/2-quart saucepan. Add onion, garlic, and a pinch of salt, then simmer, uncovered, stirring occasionally (add more water if necessary to cover chiles), until softened, about 30 minutes. While chiles are cooking, finely grind cumin seeds with a mortar and pestle or in an electric coffee/spice grinder. Transfer chiles and vegetables with a slotted spoon to a blender and purée with cumin and cup chile water (save remaining chile water) until smooth. (Sauce should be thick but not stiff; add more chile water, 1 tablespoon at a time, if necessary.) Season with salt. Cooks' note:Sauce may be made 3 days ahead, cooled completely, and chilled, covered. Reheat before serving.