Place tomatillos, serrano peppers, jalapeños, onion and garlic in a pot and cover with water. Bring to a boil then reduce to a simmer for 5-8 minutes or until the tomatillos have turned an olive green and their skins just start to split.
Remove the tomatillos, garlic and onions to a blender. Cut the stems off of the jalapeños and add into blender (remove seeds if you prefer less spicy). Add 1 cup of cooking liquid and the rest of the ingredients and blend until smooth.