Remove husks from tomatillos and rinse well until no longer sticky. Slice into quarters and place in food processor.
Add onion, serrano peppers, salt, garlic, lime juice and cilantro. Puree until smooth.
Place sauce in a medium sauce pan and simmer for approximately 15 minutes, stirring occasionally, until slightly thickened. Skim foam off the top while simmering.
Let cool, then store in refrigerator in pint-sized jars.