Mexican Spicy Tomatillo Sauce

Place tomatillos and serrano peppers into a medium pot and cover with water. Bring to a boil; reduce heat to medium-low and let simmer until tender, 15 to 20 minutes. Remove from heat, drain, and let cool, 15 to 20 minutes.

Place cooled tomatillos and serrano peppers, onion, sour cream, cilantro, avocado, garlic, salt, and pepper into an electric blender and puree until almost liquid.