In another saucepan bring the water to a boil, add the whole tomatillos and serranos, and salt lightly. Reduce heat and simmer until tender, about 15-20 minutes.
Puree in a blender with the chopped cilantro. Saute onion in lard until transparent. Pour in the soup, bring to a boil, and cook for 10 minutes.

When ready to serve, pour the potato soup into each flat soup bowl in equal portions, not filling too high. Carefully pour a scant cup of the tomatillo soup directly in the middle of each bowl of potato soup and swirl through. Sprinkle the chopped red pepper in the center of each and serve immediately.
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