Peel and wash the tomatillos, then cut each in half.
Chop the onion, garlic, chile serranos, and cilantro.
Cut the avocado in half and remove the pit.
Scoop the avocado out with a spoon.
Put above ingredients in a food processor or blender and blend.
Add the milk and water.
Salt and pepper to taste.
Purée until everything is incorporated and smooth (if the mixture is too thick, add a little more water).