After you've washed and chopped your vegetables, bring a small pot of water to a boil.
Add tomatillos and serrano peppers to pot of boiling water. Boil until tomatillos turn brown, but remain green on top, about 10 minutes.
Save 1/4 cup of water from the pot, remove vegetables and add to blender. Allow to cool for about 5 minutes.
Add raw garlic clove, cilantro, avocado, green onion, salt, and the saved water to the blender.
Blend until smooth.
Pour the salsa verde into a bowl for serving and allow to cool to room temperature.
