I've replaced the finely chopped onion with finely chopped shallot and it's fine, but I wouldn't suggest leaving it out
The original recipe has 1 pint of tomatoes, but I think that's way too much. Definitely use grape tomatoes though, since they have a lower water content and won't dilute the guacamole
Instructions
In a bowl, combine 2 tablespoons of the cilantro, the chilies, onion, and 1/2 teaspoon of salt. Mash with the bottom of a dry measuring cup until a rough paste forms, about 1 minute
Scoop the avocado flesh into the bowl and coarsely mash it with a potato masher or fork. Stir in half the tomatoes until combined. Taste and season with salt
Transfer to serving bowl and sprinkle with the remaining cilantro and tomato