In a medium bowl, place 2 tablespoons of the chopped onion, the chiles, 1 tablespoon of the cilantro and the salt. With the back of a wooden spoon, mash until the mixture becomes a juicy paste. Add avocados; with a fork, coarsely mash avocados; stir to combine with the paste. Fold in the tomatoes, lime juice and remaining onion and cilantro. Serve with tortilla chips, if desired.