Remove paper husk and wash off sticky coating from tomatillos. Place tomatillos, onion, garlic and serrano chile peppers in a medium pot and cover with cold water.
Bring liquid to a boil, reduce heat and simmer for 8 minutes.
Remove from liquid and set a side to cool.
Place tomatillos, onion, garlic, serrano chile peppers, salt, lime juice and agave in a food processor.
Blend until very smooth.
Add cilantro and pulse a few times until well combined.