Put tomatoes, cilantro, onions, and garlic in a bowl and smash with a pestle (if you do not have a pestle, try with a spoon, potato masher, or a meat tenderizing hammer).
Add lime juice and salt. Mix and let sit for about 5 minutes.
Scoop the avocado pulp and add to the bowl. Mash the avocado to desired consistency (do you want it chunky? Smooth?).