In saucepan, fill water to 2 inches and boil. Add husked tomatillos and jalapeno. Cover and steam for 5-7 minutes.
Add tomatillos, jalapeno, and 2 tablespoons of the cooking liquid to a food processor or blender. Add avocado, garlic, cilantro and salt. Blend until smooth. Add more cooking liquid to thin salsa, if desired.
Cool to room temperature to serve, or store in refrigerator up to two days. Best if served same day.
* I found that 1/2 jalapeno with the seeds was the perfect amount of mild heat for me