Put the onions, half the chiles and half the cilantro in a mortar or molcajete and use a pestle to crush slightly and form a chunky paste.
Slice the avocados lengthwise, remove the pit (but don't throw out), slice in a criss-cross manner with the tip of a knife and scoop out the flesh on top of the paste. Add a bit of salt, squeeze the lime and mash all together so it's all blended but slightly chunky. Taste, add some more chile and salt to your liking, and add the remaining cilantro, mashing slightly to blend everything together. If using tomatoes, stir in at this point. Place the pit of the avocado in the center to prevent the guacamole from browning if you aren't going to serve it right away, and just toss the pit once you do serve.
Cook's Note: A molcajete is a mortar made from volcanic rock used in many traditional Mexican recipes.
This recipe was provided by professional chefs and has been scaled down from a bulk recipe provided by a restaurant. The Food Network Kitchens chefs have not tested this recipe, in the proportions indicated, and therefore, we cannot make any representation as to the results.