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. About 12-15 minutes.
Using a blender, puree tomatillos and water in batches until smooth.