Place tomatillos, onion, garlic, and chile pepper into a saucepan. Season with cilantro, oregano, cumin, and salt; pour in water. Bring to a boil over high heat, then reduce heat to medium-low, and simmer until the tomatillos are soft, 10 to 15 minutes.
Using a blender, carefully puree the tomatillos and water in batches until smooth.
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