Place the tomatillos, serrano peppers, onion, and garlic in a saucepan, and add water to just cover. Sprinkle the salt over the top, bring to a boil, reduce the heat to medium-low, and cook until the tomatillos are soft and have turned slightly brownish in color, 20 to 30 minutes. Add more water if needed to keep the mixture from burning as it cooks.
Pour the cooked vegetables into a blender, and blend until smooth.
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