Place the onion, jalapeño, salt and half the coriander on a cutting board and use a fork to mash until juicy. OR do this in a mortar and pestle - grind into a paste.
Scrape into a bowl, add avocado and remaining coriander, and mash to desired consistency.
Do a taste test then adjust to your taste: salt, lime juice for sour, more chilli for spiciness.
If using tomatoes, stir through.
Serve with corn chips!
