Add the tomatillos, garlic, Serrano chiles, onion, cilantro, salt and cumin into a blender. Blend until it has a smooth consistency.
In a saucepot on medium-low heat, heat up the oil. Pour the blended salsa into the pan and cook while occasionally stirring for 10 minutes. Taste the salsa and add salt and adjust the seasonings. Can serve hot or cold.