Place tomatillos, onion, serrano, and garlic in a large saucepan; season with cilantro, oregano, salt, and cumin. Add water; bring to a boil over high heat. Reduce the heat to medium-low; simmer until tomatillos are soft, 10 to 15 minutes.
3
Transfer tomatillo mixture to a blender in batches; blend until smooth.
