Slit the avocado horizontally. When your knife kits the stone inside, turn the avocado around as you cut so that you are cutting around the stone. In short, the slit which goes all the way to the stone should go around the avocado as well so that you have two halves which are still connected by the stone. Twist the two halves to expose the stone. Remove the stone. Scrape off the fibrous center when the stone was. Scoop out the avocado flesh and into a bowl. Mash lightly with a fork.
Halve the tomato. Scoop out the seeds and discard. Roughly chop the flesh.
Peel and chop the red onion.
Finely slice the chilies.
Add the tomato, onion, chilies and cilantro to the avocado in the bowl.