Place onion, pepper, 1/4 cup cilantro and salt in a mortar and pestle or a small food processor. Pound or process into a fine paste.
Scoop out the flesh of the avocados using a spoon into a medium bowl. Roughly mash with a fork. Add the onion paste, remaining cilantro and lime juice. Fold to combine. Taste and add additional salt, lime juice and any optional toppings and seasonings. Serve immediately with warm tortilla chips.