In a small saucepan of boiling water, cook the tomatillos until just tender, about 3 minutes. Drain and cool. Coarsely chop the tomatillos.
In a food processor, process the tomatillos for 5 seconds. Add the onion and process to a coarse puree. Transfer the sauce to a bowl and stir in the garlic and cilantro. Just before serving, season with salt.