Place the tomatillos, garlic clove, and chiles in a medium saucepan, cover with water, and bring to a boil over medium-high heat. Reduce the heat to medium and simmer for 10 to 12 minutes, until the tomatillos are mushy and very soft but have not begun to break apart.
With a slotted spoon, transfer the tomatillos, chiles, and garlic to a blender. Add the onion, cilantro, avocado, and salt and puree until smooth.
Serve at once, or transfer to a container, placing a sheet of plastic wrap directly against the surface before topping with the lid to keep the nice green color. The salsa can be refrigerated for a couple of days. Stir before you use it.