Put the diced onion, minced jalapeno, half the cilantro and the salt into a molcajete or a sturdy bowl. Grind or mash them together into a rough paste.
Halve and pit the avocados, then scoop the flesh into the bowl.
Fold the avocado through the paste with a fork or the back of a spoon, pressing only enough to break it into large lumps. Stop while it is still visibly chunky.
Add the lime juice and fold once or twice more.
Fold in the diced tomato and the remaining cilantro last, with the lightest hand, so the tomato stays intact and does not water the mixture down.
Taste and adjust the salt and lime. Serve within an hour for the best colour.