Place chiles on a foil-lined baking sheet; broil 14 minutes or until blackened and charred, turning after 7 minutes. Place in a heavy-duty zip-top plastic bag; seal. Let stand 15 minutes. Peel and discard skins. Cut a lengthwise slit in each chile. Remove and discard seeds and tops. Chop chiles to measure 3/4 cup.
Heat canola oil in a medium saucepan over medium-high heat. Add onion to pan; sauté 5 minutes or until tender. Add garlic; sauté for 1 minute. Stir in flour; cook 1 minute. Add chiles, coriander, salt, and broth; bring to a boil. Reduce heat, and simmer 15 minutes, stirring occasionally. Place half of chile mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until smooth. Return pureed chile mixture to pan; stir well. Remove from heat; cool completely.
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