For a milder puree, rub chiles between your palms to remove most
of the seeds. Chop chiles coarsely with a knife or in a food processor.
Put them in a bowl and pour the 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