For a milder sauce, remove the seeds from the jalapeño before processing. Serve this on the side with the chicken or flank steak.
Combine tomatillos, onion, and water to cover in a small saucepan. Bring to a boil, and cook 5 minutes. Turn off heat, and let stand 20 minutes or until tomatillos are tender. Drain.
Process tomatillo mixture, 1/2 cup water, and remaining ingredients in a blender until smooth. Cover and chill until ready to serve.
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