Place the chile, onion, and cilantro into the molcajete*. Crush and mix the ingredients together. This brings out the heat of the chile, the flavors of the fresh cilantro, and the natural juices of the onion. Score the avocado halves with the tip of the spoon and scoop all the flesh into the molcajete. Mash the avocado and combine it with the muddled onion, cilantro, and chile. Top the mixture with the diced tomato and serve with the warm flour tortillas or corn tortilla chips.
*A molcajete is a traditional Mexican mortar and pestle made from porous basalt volcanic rock. Another type of mortar and pestle may also be used.