Lay whole peppers on their sides on racks of gas stove burners (preferably 1 to a burner) and turn flame on high. Roast peppers, turning them, until skins are blistered and have brown patches, 3-6 minutes. Do not overchar peppers, they will lose their bright color. (Alternatively, peppers may be roasted in same manner on rack of a broiler pan under a preheated broiler about 2" from heat). Transfer peppers to a bowl and let stand, covered, until cool enough to handle. Peel peppers (wear rubber gloves when handling chiles) Cut off tops and discard seeds and ribs. Chop peppers. (Peppers may be prepared one day ahead and chilled covered.)
Chop garlic and chop enough onion to measure 1 cup. In a nonstick skillet cook garlic and onion in oil over moderately low heat, stirring, until sofened. Skim fat from broth. Stir in broth and peppers and simmer 1 minute. In a blender, puree mixture until completely smooth (use caution when blending hot liquids) and season with salt.
Makes 2 cups.
See also Cilantro Stuffed Chicken