In a small saucepan, combine tomatillos (3/4 pound), jalepno, and enough water to cover ingredients by 1 inch. Bring to a boil over mid-high. Reduce to a rapid simmer and cook until vegetables are tender, 7 min. With a slotted spoon, transfer to a blender and add onion. Puree until smooth. Add cilantro, buttermil and avocado. Season with salt, puree until smooth.
Can substitute greek yogurt for buttermilk for a creamier texture/flavor.
