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.
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Notes
For less heat, feel free to substitute fresh jalapeños or Anaheim chiles for the Serrano chiles.