In a medium-size bowl, mix the finely chopped onion and chiles with the optional tomato, garlic, and cilantro.
Close to the time you are going to serve, halve the avocados lengthwise by cutting from the stem to flower ends, around the pits. Twist the avocado halves in opposite directions to loosen the meat from pits, then scoop out the pits, and reserve. Scrape the avocado pulp from the skins and add it to the bowl.
Using your hand or a spoon, roughly mash the avocado while mixing in the other ingredients, making a course, thick mass. Flavor with salt, then enough lime juice to add a little zing, if you wish. Return the pits to the guacamole and cover with a sheet of plastic wrap pressed directly onto the surface of the mixture. Set aside for a few minutes to let the flavors blend.
The guacamole is very attractive in a pottery bowl or Mexican mortar, sprinkle with chopped onion, cilantro, radish slices and/or queso fresco (cheese); radish roses really dress it up.
Recipe courtesy of Rick Bayless; From Authentic Mexican, Morrow 1987.