To a medium saucepan, add the tomatillos and chiles, and add enough water to cover them. Bring to a boil and cook 2-3 minutes. Carefully place tomatillos and chiles in a food processor bowl, reserving cooking water.
Pulse together cooked tomatillos, chiles and garlic until blended. Add avocado and pulse until blended. Add a little of the cooking liquid to thin to a salsa consistency. Taste and season with salt.
At this stage, the guacamole salsa will be warm. If you prefer, chill salsa in refrigerator for 30 minutes.
Transfer to a serving bowl and top with diced onion and chopped cilantro. Serve with tortilla chips and limes (optional) to squeeze on top.
For less heat, feel free to substitute fresh jalapeños or Anaheim chiles for the Serrano chiles.