Roast the tomatillos by placing the whole fruit in a hot skillet or cast-iron griddle over medium heat and roast, turning occasionally, until quite charred all over and slightly softened.  Combine the onion, chilies, cilantro, and salt in a large bowl.  Add the tomatillos, a few at a time, mashing and blending with a fork or pestle to a fine paste.  Add the avocados and continue mashing and mixing until chunky.  Serve immediately as a dip with slices of ripe tomato or tortilla chips.