Husk and wash the fresh green tomatillos. Pat dry, stem and chop them. Chop the Serrano chiles. In a blender, puree the tomatillos, chiles, cup minced white onion, garlic and fresh cilantro until well-blended and smooth.
In a large serving bowl, pour mixture and stir in the remaining minced onion. Season to taste with salt. Refrigerate overnight or serve immediately with fresh corn chips. Score!