1-2serrano chiles or other fresh green chiles, finely chopped
juice of 1-2 limes
a small handful fresh cilantro, chopped
Put a quarter of the onion and half the chili and salt in a pestle and mortar and mash to a rough paste.
Cut open the avocados, remove the stones and scoop out the flesh into the pestle (or into a large bowl if your pestle is small). Roughly mash the flesh with a fork, adding half the lime juice as you go.
Stir in the rest of the lime juice and chilies, coriander, red onion and tomato. Season with plenty of black pepper and salt (only if more salt is needed).