Boil tomatillos, garlic and Serrano pepper for 10 minutes. (They are done when they change color)
Finely chop onion and marinate in lime juice for 5-10 minutes. (This cooks the onion in acid.)
Put the tomatillos and peppers into a food processor and pulse 2 times.
Add avocado, cilantro, onion/lime juice, serrano pepper, and salt.
Process until smooth.
Taste and adjust seasoning, then transfer to a serving bowl.
