In a medium bowl, add the avocado halves. Mash with a fork or potato masher until you reach your desired consistency—leave it a little chunky for texture.
Add the diced onion, garlic, tomatoes, 3 tablespoons (45 ml / 1.5 fl oz) of lime juice, cilantro, salt, and cumin to the bowl.
Stir gently until everything is well combined.
Taste and adjust seasoning, adding the remaining tablespoon of lime juice or more salt if needed.
Serve immediately or cover with plastic wrap pressed directly onto the surface of the guacamole and refrigerate for up to 1 hour to let the flavors meld.