In a mixing bowl, combine tomatoes, onion, garlic, and jalapeño (if using).
Slice open avocados, remove pit, cut into cubes, and add into the mixing bowl. Squeeze in lime juice to help counteract the oxidation (browning) of the avocado. Mash avocado with a fork to get desired consistency leave some in larger chunks for texture.
Sprinkle evenly with salt, pepper, and cumin. Toss everything together until it is evenly combined. Taste and season with additional seasonings and/or lime juice if desired. Add cilantro and gently toss to combine. Serve immediately or chilled.