Put a quarter of the onion and half the chilli 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 mortar (or into a large bowl if your mortar is small).
Roughly mash the flesh with a fork, adding half the lime juice as you go. When you have a rough guacamole, stir in the rest of the lime juice, red onion and chilli and the coriander.
Season with plenty of black pepper and more salt if you think it needs it. If it doesn't taste delicious by this stage, think whether it might need more lime juice or coriander.
Serve the guacamole with freshly made tortilla chips or pork scratchings.