Creamy Tomatillo Sauce

In a small saucepan, combine tomatillos, jalapeno, and enough water to cover ingredients by 1 inch. Bring to a boil over medium-high. Reduce to a rapid simmer and cook until vegetables are tender, 7 minutes. With a slotted spoon, transfer to a blender and add onion. Puree until smooth. Add cilantro, buttermilk, and avocado. Season with coarse salt; puree until smooth.