Combine tomatillos, chile and cilantro in a blender jar or food processor. Add 1/4 cup water and process to a coarse puree. If using a blender, begin blending on low, at first pulsing to get the mixture moving evenly through the blender blades. Add avocado and blend again until homogenous. Pour into a salsa dish and mix in rinsed white onion. Thin with a little more water if necessary to give the salsa an easily spoonable consistency. Taste and season with salt. Serve right away.