Cut avocados in half, remove seeds & skin, cut into chunks & set aside. Remove stems from Serrano peppers, cut into thirds & set aside.
Peel and wash the tomatillos - they are usually found over by the cilantro & peppers, and have a husk on them. Cut them into quarters and put in a blender.
Follow, in order, with the serrano peppers, cilantro, onion, garlic, avocado, lime juice & salt.
Be sure to put the tomatillos in the bottom of the blender and not the avocado so it doesn't stick to the blade. You might have to help it start by shaking or using a mixer at first. But, once it gets going the juice from the tomatillos will help move the other ingredients around.
Once blended, scrape salsa out of blender into a jar or container with a lid.
Optional: Mix in some sour cream to taste.
Keep in a sealed container in the refrigerator. Should keep for about a week!