For a milder puree, rub chiles between your palms to remove most
of the seeds. Chop chiles coarsely with a knife or in food processor.
Put them in a bowl and pour vinegar over. Cover and refrigerate
Add onion and garlic to the chiles and either cover the bowl with
plastic and microwave on HIGH for 1 minute, or put them in a saucepan
over medium-high heat; cover, and simmer for 3 minutes. Let cool
slightly, then puree in a blender until very smooth. Let cool.